Biological classification

“ Conversely, what a strong support of the evolution principle is afforded by the fact that the two systems of classification point to the same lines of descent!
Evidences from Embryology
There should be no sharp division between the evidences from comparative anatomy and those from embryology. Those two branches of biology are inseparable; one must be interpreted in the light of the other. Comparative anatomy deals with the adult structures of organisms. ”

“ The result attained in attempting to exhibit the relationships of the mere forms of organisms by a tabular classification is only explicable when regarded as the expression of their actual blood relationship; the tree shape of the natural system can only be understood as the actual pedigree of the organisms. ”

“ It is very important to select the right animals for such investigations. I can conceive that a lifetime should be passed in embryological studies, and yet little be learned of the principles of classification. The embryology of the worm, for instance, would not give us the natural classification of the Articulates, because we should see only the first step of the series; we should not reach the sequence of the development. ”

“ On this idea of the natural system being, in so far as it has been perfected, genealogical in its arrangement, with the grades of difference expressed by the terms genera, families, orders, etc., we can understand the rules which we are compelled to follow in our classification. We can understand why we value certain resemblances far more than others; why we use rudimentary and useless organs, or others of trifling physiological importance ”

“ It is however becoming probable that if more knowledge of the chemical and physical structure of organisms is to be attained, the clue will be found through Genetics, and thus that even in the uncoordinated accumulation of facts of Variation we are providing the means of analysis applicable not only to them, but to the problems of normality also.
The only classification that we can yet institute with any confidence among the phenomena of Variation is that which distinguishes on the one hand variations in the processes of division from variations in the nature of the substances divided. ”

“ It is only after exclusion that the peculiarities of the structure of the teeth and fins indicate the genus and species” (§ 319) . “Hence the embryos of different animals resemble each other the more, the younger they are” (§ 320) . “Consequently the high importance of developmental history is indubitable. For, if the formation of the organs takes place in the order corresponding to their importance, this sequence must of itself be a criterion of their comparative value in classification. ”

“ We can understand why a classification founded on any single character or organ—even an organ so wonderfully complex and important as the brain—or on the high development of the mental faculties, is almost sure to prove unsatisfactory. This principle has indeed been tried with hymenopterous insects; but when thus classed by their habits or instincts, the arrangement proved thoroughly artificial.257 Classifications may, of course, be based on any character whatever, as on size, colour, or the element inhabited; but naturalists have long felt a profound conviction that there is a natural system. ”

“ This, of course, is purposely chosen as a very simple illustration; but it exemplifies the general fact that thepg. 23 guiding principle of scientific classification is the comparing of organism with organism, with the view of seeing which of the constituent organs are of the most invariable occurrence, and therefore of the most typical signification. ”

“ And it may be further remarked, that the extent to which this classification is carried, roughly indicates the height of intelligence—that while the lowest organisms are able to do little more than discriminate organic from inorganic matter; while the generality of animals carry their classifications no further than to a limited number of plants or creatures serving for food, a limited number of beasts of prey, and a limited number of places and materials ”

“ When we look beyond this large and general fact of all the innumerable forms of life being thus united in a tree-like system by an unquestionable relationship of some kind, to those smaller details in the science of classification which have been found most useful as guides for this kind of research, then we find that all these details, or empirically discovered rules, are exactly what we should have expected them to be, supposing the real meaning of classification to have been that of tracing lines of pedigree. ”

“ But so firmly is the belief in the correspondence of ontogeny and phylogeny fixed in the minds of modern biologists that, in determining the classification and affinities of any particular animal, more reliance is placed on the facts of embryology than on those of adult structure. ”

“ The most instructive classification of the “variations” exhibited by fully formed organisms consists in the separation in the first place of those which arise from antecedent congenital, innate, constitutional or germinal variations from those which arise merely from the operation of variation of the environment or the food-supply upon normally constituted individuals. ”

“ It has turned out that the earlier classifications, guided by general resemblances, though containing much truth, and though very useful provisionally, were yet in many cases radically wrong; and that the true affinities of organisms, and the true homologies of their parts, are to be made out only by examining their hidden structures. Another fact of great significance in the history of classification is also to be noted. Very frequently the kinship of an organism cannot be made out even by exhaustive analysis, if that analysis is confined to the adult structure. ”

“ In the disentanglement of the properties and interactions of these elementary factors, the science we must call to our aid is Physiological Chemistry. The relations of Genetics with the other branches of biology are close. Such work can only be conducted by those who have the good fortune to be able to count upon continual help and advice from specialists in the various branches of Zoology, Physiology, and Botany. ”

“ If we could conceive of an organism like one of those to which disease may be due becoming actually incorporated with the system of its host, so as to form a constituent of its germ-cells and to take part in the symmetry of their divisions, we should have something analogous to the case of a species which acquires a new factor and emits a dominant variety. ”

“ It has always been the endeavor of students of nature to reduce this great host of living creatures to order by some system of classification. First one system of classification was adopted, and then another, with the progress of Botany and Zoölogy, until eventually, before the theory of evolution was entertained by scientific men, that system was adopted which naturalists likened to a tree. In this system, those lowest organisms which cannot properly be called either animals or plants, may be represented by a short trunk. ”

“ The structural changes which any series of individual organisms exhibits, admit of similar classification. On the one hand, we have those inner and outer differences of shape, that arise between the adult members of successive generations descended from a common stock—differences which, though usually not marked between adjacent generations, become great in course of multitudinous generations. On the other hand, we have those developmental modifications, seen in the embryos, through which such modifications of the descended forms are reached. ”

“ Not only the established truths of Biology, but the established truths of science in general, negative the supposition that organisms having structures definite enough to identify them as belonging to known genera and species, can be produced in the absence of germs derived from antecedent organisms of the same genera and species. ”

“ If we clearly recognize that the physical basis of life is the same in both plant and animal, and that it is through disturbance, primarily, of the protoplasmic functions that the functions of the organism as a whole are disordered, then from this and what has already been stated it becomes evident that the pathology of plants and animals is the same, chiefly involving greater degrees of complexity in the latter, and that we must apply the same principles for the recognition of disease in each case. ”

“ Thus Bionomics is treated in such articles as Evolution, Heredity, Variation, Mendelism, Reproduction, Sex, &c.; Zoo-dynamics under Medicine, Surgery, Physiology, Anatomy, Embryology, and allied articles; Plasmology under Cytology, Protoplasm, &c.; and Philosophical Zoology under numerous headings, Evolution, Biology, &c. ”

“ Darwin accepted such transmission as subordinate to natural selection, and attempted to account for it by his theory of pangenesis. According to that hypothesis all the component cells of an organism throw off minute gemmules, and these and their like, collecting in the reproductive cells, are the parental germs from which all the cells of the offspring of that organism are developed. ”

“ On the other hand features of structure, sexual differentiation, and other attributes which by our standards are estimated as essential, may be declared to show much variation or little, not according to any principle which can be detected, but simply as the attention happens to be applied to one species or group of species, or to another. In many groups of animals and plants observers have hit upon characters which were for a time thought to be finally diagnostic of species. ”

“ Biogeny, or the science of the evolution of organisms, is also divided into two parts—the science of the individual (ontogeny) and of the stem or species (phylogeny) ; each follows its own peculiar methods and aims, but they are most intimately connected by the biogenetic law. Ontogeny deals with the development of the individual organism from the beginning of its existence to death; as embryology it observes the growth of the individual within the fœtal membranes ”

“ I may in one respect even go further, and affirm, that, if in a species, any part or organ differs remarkably from the same part in its congeners, then if many specimens are examined, especially when collected from different districts, such part or organ will be found eminently variable. ”

“ It is clear that the organism that best uses its organs has, other things equal, the best chance of survival. It will therefore hand on to its offspring germinal matter with an inherent tendency to make vigorous use of its faculties.
Those who argue thus deny that the body-cells can in any way affect the germ-cells. To account for any continuous increase in faculty, they invoke variation and the selection of favourable varieties. What, then, we may now ask, is, on their view, the mode of origin of variations? ”

“ Not till knowledge of the genetic properties of organisms has attained to far greater completeness can evolutionary speculations have more than a suggestive value. By genetic experiment, cytology and physiological chemistry aiding, we may hope to acquire such knowledge. ”

“ Biology uses experiment and hence begins with germ cells: social science is based on observation and hence its data are the visible differences which the study of men affords. Both call their data characters, but as a loose terminology creates confusion, I shall call the germinal variations the biologist finds through experiment characters, while the visible differences in men open to observation I shall call traits. The problem then is what is the relation of biologic characters to visible traits? Is there a character for each trait, or do independent laws govern traits? ”

“ Very recently evidence has, indeed, been adduced to prove that "Initiative in animal evolution comes by stimulation, excitation and response in new conditions, and is followed by repetition of these phenomena until they result in structural modifications, transmitted and directed by selection and the law of genetics." ”

“ The neglect is due partly to the erroneous opinion that not the physiological function, but only its substratum the bodily organ, is capable of evolution. The organ, that much is certain, develops—that is, it passes through a series of transformations before assuming its final form. Any part of every organism will furnish the proof. But what determines the final form during phylogenetic evolution? I answer, Function. With its assertion begins the differentiation of the substratum of primitive beings. It is not the organ from which function derives its origin, but just the reverse. ”

“ His more immediate effort is to inquire into the nature of the existing organism, to ascertain in what measure the complex phenomena of life as they now appear are capable of resolution into simpler factors or components, and to determine as far as he can what is the relation of these factors to other natural phenomena. It is often practically convenient to consider the organism as presenting two different aspects—a structural or morphological one, and a functional or physiological—and biologists often call themselves accordingly morphologists or physiologists. ”

“ If the catalogs are poorly made,one may draw no conclusions or, what is worse, draw conclusions as fantastic as the hobgoblins of primitive imagination. But if the taxonomic arrangement brings together species of common ancestry and accurately portrays the varying degrees of relationship between those species, a classification becomes one of the most powerful tools available for the evolutionary interpretation of biologic phenomena. ”