Biological hierarchy

“ Finally, as rudimentary organs, by whatever steps they may have been degraded into their present useless condition, are the record of a former state of things, and have been retained solely through the power of inheritance,— we can understand, on the genealogical view of classification, how it is that systematists, in placing organisms in their proper places in the natural system, have often found rudimentary parts as useful as, or even sometimes more useful than, parts of high physiological importance. ”

“ My present task is not to deal with the whole question of heredity, but only with the single although fundamental question—‘How is it that a single cell of the body can contain within itself all the hereditary tendencies of the whole organism?’ I am here leaving out of account the further question as to the forces and the mechanism by which these tendencies are developed in the building-up of the organism. ”

“ It results that the multiplied repetitions of these acts of organization strengthen, extend, develop, and even create the organs which are necessary. It is only necessary attentively to observe that which is everywhere occurring to convince ourselves of the well-grounded basis of this cause of organic developments and changes.
“Moreover, every change acquired in an organ by a habit of use sufficient to have produced it is then preserved by heredity (génération) if it is common to the individuals which, in fecundation, unite in the reproduction of their species. ”

“ In plants causations are organized as generations as well as forces and forms and kinds. In animals concepts are organized as well as causations, forces, forms and kinds. In every one of these systems there is a special differentiation and integration of organs; so the entire body is organized in a hierarchy of organs. ”

“ In the course of the phyletic development of the organized world, it must have happened that certain unicellular individuals did not separate from one another immediately after division, but lived together, at first as equivalent elements, each of which retained all the animal functions, including that of reproduction. ”

“ Here, then, we have the great principle of the division of labour, which is the principle of all organization, taken as primarily illustrated in the division between the reproductive cells and the non-reproductive or somatic cells—the cells devoted to the continuance of the species, and the cells which subserve the life of the individual. ”

“ In this manner we can explain the origin of hereditary individual variability as it is known in man and the higher animals, and as it is required for the theory which explains the transformation of species by means of natural selection. ”

“ Resting upon the law of the division of physiological work, the importance of which was first demonstrated by Milne-Edwards, and upon that of polymorphism, which without it have only a limited and indefinite meaning, consequent on the law of division of protoplasmic masses, it has been the great producer of organization, and establishes a new link between sociology and the branches of biology that are occupied with the constitution and functions of organisms. ”

“ Our belief that a species produces only its own species, that like begets only like, a belief that finds continual confirmation all through the study of systematic and embryological natural history, would disappear, were it possible that in the division of unicellular organisms the hereditary mass should be split into two unequal components and be bestowed unequally upon the daughter-cells. ”

“ Nor are we likely to forget, in the midst of our commemoration of Darwin's birth and of "The Origin of Species," that the whole doctrine of organic evolution rests upon the facts of heredity. Whatever the factors that determine racial descent—fluctuating variation, or the sudden change of type, use and disuse, natural, artificial, sexual or organic selection—both continuity in the process of bionomic change and maintenance of the change once produced, demand the conception of a hereditary likeness. ”

“ It therefore follows that every modification which appears in the course of its life, every individual character, however it may have arisen, must necessarily be directly transmitted to the two offspring of a unicellular organism. ”

“ Superfluous organs become rudimentary very slowly, and enormous periods must elapse before they completely disappear, while the older a character is, the more firmly it becomes rooted in the organism. What I have above called the physical constitution of a species is based upon these facts, and upon them depend the tout ensemble of inherited characters, which are adapted to one another and woven together into a harmonious whole. ”

“ Impossible though it may be to imagine the natures of those ultimate units peculiar to each species, which have proclivities towards the particular form of organization characterizing it, yet that a change of structure arising in one part of the organism is accompanied by multitudinous changes of structure in other parts of the organism, is not only congruous with the belief that there exist such constitutional units, but yields it distinct support. ”

“ But since there is a universal principle of development, namely, the formation of cells, does it not seem that the cells must be the true 182organisms, that the whole "individual" organism must be an aggregate of cells, and that the concept of individuality applied to the organism is accordingly a logical fiction? ”

“ Let two forms have not a single character in common, yet, if these extreme forms are connected together by a chain of intermediate groups, we may at once infer their community of descent, and we put them all into the same class. As we find organs of high physiological importance — those which serve to preserve life under the most diverse conditions of existence — are generally the most constant, we attach especial value to them; but if these same organs, in another group or section of a group, are found to differ much, we at once value them less in our classification. ”

“ A system, in order to be true and natural, must agree with the sequence of the organs in the development of the embryo.” If the earlier manifested peculiarities are to be estimated more highly than those which afterwards make their appearance, then in those cases in which the structure of the adult animal requires one position in the system, and that of the larva another, the latter and not the former must decide the point. ”

“ The doctrine of typical plans is equally out of court; for while, in some members of a group, rudimentary organs completing the general type are traceable, in other members of the same group such organs are unrepresented. There remains only the doctrine of evolution; and to this, these rudimentary organs offer no difficulties. ”

“ The proofs that are relied upon to establish the operation and effect of these agencies in producing the results that are claimed for them, ought to show that, in one or more instances, an animal of a superior organization which, when left to the natural course of its reproduction by the union of its two sexes, always produces its own distinct type and no other, has, in fact, been itself evolved out of some lower and different organism by the agencies of natural and sexual selection operating among the individuals of that lower type. ”

“ First, we have to consider the relative place which comparative anatomy concedes to man in the 'natural system' of animals, for the true value of our 'natural classification' is based upon its meaning as a pedigree. All the minor and major groups of the system—the classes, legions, orders, families, genera, and species—are only different branches of the same pedigree. For man himself, his place in the pedigree has been fixed since Lamarck, [5] in 1801, defined the group of vertebrates. ”

“ Just as we had employed all the comparisons that pedigree furnishes to express the likenesses among organisms before supposing them really to be blood relations, so we have compared organisms to societies and societies to organisms, all the while regarding these comparisons as mere fancies. On the contrary, in the last year, we have reached the conclusion that association has played an important if not exclusive part in the development of organs. ”

“ The individual development or ontogenesis of each of these many-celled animal-forms, brings this histological process of development so clearly and evidently before our eyes that we can but directly infer from it the truth of the phylogenesis, or gradual historical evolution of the soul-organs. The association of cells and the division of labour among them are the modes by which, in the first instance, the compound many-celled organism has originated, historically, from the simple one-celled organism. ”

“ On these same principles,— and bearing in mind that when organs are reduced in size, either from disuse or through natural selection, it will generally be at that period of life when the being has to provide for its own wants, and bearing in mind how strong is the force of inheritance — the occurrence of rudimentary organs might even have been anticipated. The importance of embryological characters and of rudimentary organs in classification is intelligible, on the view that a natural arrangement must be genealogical. ”

“ We can readily conceive that, as the process of specialization is going on, modifications or variations in groups of cells and the tissues derived from them, notwithstanding the influence of heredity, may in an individual diverge so far from that which is characteristic of the species as to assume the arrangements found in another species, or even in another order. ”

“ For these he depends on the labours of other specialists; and therefore the higher the state of civilization the more complete is the state of interdependence of the members of the community.
So in higher animal organisms we find that all cells have taken up special duties—have all become specialists. Every kind of cell performs one function specially well and the others not so well, or not at all. ”

“ The two chief factors which play a part in the origin of species are heredity and variation, and until we know more about the laws which govern these factors, we cannot hope to arrive at any satisfactory criteria by which we can estimate the importance of the data accumulated for us by comparative anatomists and embryologists. ”

“ Lastly, the fact, that no one character is of so much importance in determining to what great group an organism belongs, as the forms through which the embryo{433} passes from the germ upwards to maturity, cannot be reconciled {201} with the idea that natural classification follows according to the degrees of resemblance in the parts of most physiological importance. ”

“ On this condition only does the process of organization appear possible. We might almost say that just as some race of animals, which multiplies and spreads into divers regions of the earth, becomes differentiated into several races through the adaptation of each to its conditions of life; so, the originally homogeneous population of cells arising in a fertilized germ-cell, becomes divided into several populations of cells that grow unlike in virtue of the unlikeness of their circumstances. ”

“ It is a mistake to attempt to explain all the results of evolution by a principle. There are two kinds of congenital, constitutional or hereditary characters in all organisms, namely, the adaptive and the non-adaptive, and every distinct type in classification exhibits a combination of the two. ”

“ On the contrary, if we begin with the end which comprises animals the most simple in organization, the poorest in faculties and in organs—in a word, the most imperfect in all respects—we necessarily remark, as we gradually ascend in the series, a truly progressive complication in the organization of these different animals, and we see the organs and faculties of these beings successively multiplying and diversifying in a most remarkable manner. ”

“ Hence, all the individual attempts to obtain a knowledge of the pedigree of any one group of organisms possesses but a temporary and conditional value, and any special hypothesis relating to it will become the more and more perfect the greater the advance we make in the comparative anatomy, ontogeny, and palæontology of the group in question. ”

“ This hereditary substance is the germ-plasm. It is the living, vital substance of organisms that takes part in the development of new generations.
Naturalists are agreed on this point, that the more complex animals and plants have been derived from the simpler ones; and, this being accepted, the attention should be fixed on the nature of the connection between generations during their long line of descent. In the reproduction of single-celled organisms, the substance of the entire body is divided during the transmission of life, and the problem both of heredity and origin is relatively simple. ”

“ The second great problem is, How is it that existing animals, and, as the evidence of the remains of extinct animals shows, these that existed at former periods of time also, are divided into the groups or types we call species, naturally classified into larger groups which are subdivisions of others still larger, and so on, in what we call the natural system of classification? The two problems which naturalists have to solve, and which for many recent generations they have been trying to solve, are the Origin of Species and the Origin of Adaptations. ”

“ They operate entirely within the framework of natural laws, and can only utilize what energies are available for them at the given time and place. Every cell has its dominants; and as an [Pg 122] organism is a synthesis, not a mere aggregate, of cells, so its individual dominant is a synthesis of the dominants of its parts. The evolution of species, like the development of an embryo, is under the control of dominants. ”