Herbert Spencer

“ While we recognize this truth we must also recognize the truth that only by enunciation and acceptance of imperfect generalizations can we progress to perfect ones. The order of Evolution is conformed to by ideas as by other things. ”

“ For if we follow out in thought the implications—if we conceive the germs of all kinds of organisms simultaneously developing, and imagine that after taking their first step together, at the second step one half of the vast multitude diverges from the other half; if, at the next step, we mentally watch the parts of each great assemblage beginning to take two or more routes of development ”

“ The substitutions of organs and the suppressions of organs, are among those secondary embryological phenomena which harmonize with the belief in evolution but cannot be reconciled with any other belief. Some embryos, during early stages of development, possess organs that afterwards dwindle away, as there arise other organs to discharge the same functions. And in other embryos organs make their appearance, grow to certain points, have no functions to discharge, and disappear by absorption. ”

“ If the whole series of phylogenetic changes had to be repeated—if the embryo mammal had to become a complete fish, and then a complete amphibian, and then a complete reptile, there would be an immense amount of superfluous building up and pulling down, entailing great waste of time and of materials. ”

“ Darwin says that in some genera of cirripedes, "the larvæ become developed either into hermaphrodites having the ordinary structure, or into what I have called complemental males, and in the latter, the development has assuredly been retrograde; for the male is a mere sack, which lives for a short time, and is destitute of mouth, stomach, or other organ of importance, excepting for reproduction." ”

“ That unbuilding and rebuilding which would be needful were it laid down of its original size, will be made needless if from the beginning it is laid down of a larger size. Hence, in successive generations, the greater prosperity and multiplication of individuals in which this part is at the outset somewhat larger than usual, must eventually establish a marked excess in its development at an early stage. ”

“ There is an embryology of conceptions. That this statement is not wholly a figure of speech, we shall see on considering that cerebral organization is a part of organization at large; and that the evolving nervous plexus which is the correlative of an evolving conception, must conform to the general law of change conformed to in the evolution of the whole nervous structure as well as in the evolution of the whole bodily structure. As the body has at first a rude form, very remotely suggesting that which is presently developed by the superposing of modifications on modifications ”

“ Hence, remembering the perpetual intrusions of organisms on one another's modes of life, often widely different; and remembering that these intrusions have been going on from the beginning; we shall be prepared to find that the general law of embryonic parallelism is qualified by irregularities which are mostly small, in many cases considerable, and occasionally great. The hypothesis of evolution accounts for these: it does more—it implies the necessity of them. ”

“ Where an organ is of such size that progressive diminution of it will appreciably profit the young animal, by leaving it a larger surplus of unused material, we may expect progressive diminution to occur. Contrariwise, if the organ is relatively so small that each decrease will not, by sensibly increasing the reserve of nutriment, give the young animal an advantage over others, decrease must not be looked for: there may be a survival of it even though of very ancient origin. ”

“ Ordinarily in vertebrates the fore limbs grow more rapidly than the hind limbs; but in the ostrich, in which the hind limbs or legs have to become so large while the wings are but little wanted, the leg development goes in advance of the wing-development in early embryonic stages: there is a pre-adaptation. ”

“ We have a remarkable instance of substitution in the temporary appliances for respiration, which some embryos exhibit. During the first phase of its development, the mammalian embryo possesses a system of blood-vessels distributed over what is called the area vasculosa—a system of vessels homologous with one which, among fishes, serves for aërating the blood until the permanent respiratory organs come into play. ”

“ The hypothesis of evolution, however, while it pre-supposes those kinships among embryos in their early forms which are found to exist, also leads us to expect nonconformities in their courses of development. ”

“ Again, the reduction of a superfluous part can take place only on condition that the economy resulting from each descending variation of it, is of greater importance than are the effects of variations simultaneously occurring in other parts. If by increase or decrease of any other parts of the embryo, survival of the animal is furthered in a greater degree than by decrease of this superfluous part, then such decrease is unlikely ”

“ For the increased passing up and down, a staircase wider at its lower part will be required. If now the builder, when putting up the ground floor, follows the old dimensions, then after all the stories are built, the lower part of the staircase, if it is to yield equal facilities for passage, must be reconstructed. Instead of a staircase adapted to those few stories which the original type of house had, economy will dictate a pre-adaptation of the staircase to the additional stories. ”

“ If, as any rational theory of evolution implies, the progressive differentiations of types from one another during past times, have resulted from the direct and indirect effects of external conditions—if races of organisms have become different, either by immediate adaptations to unlike habits of life, or by the mediate adaptations resulting from preservation of the individuals most fitted for such habits of life, or by both ”

“ Already I have emphasized the truth that Nature is always more complex than we suppose ( § 74a) —that there are complexities within complexities. Here we find illustrated this truth under another aspect. When seeking to formulate the arguments from Embryology, we are shown that the facts as presented in Nature are not to be expressed in the simple generalizations we at first make. ”

“ That subordination of classes, orders, genera, and species, to which naturalists have been gradually led, is just that subordination which results from the divergence and re-divergence of embryos, as they all unfold. On the hypothesis of evolution this parallelism has a meaning—indicates that primordial kinship of all organisms, and that progressive differentiation of them, which the hypothesis alleges. ”

“ Now it is clear that on tracing down each branch of the great genealogical tree, kinships would be much more manifest among the recently-differentiated forms than among those forms which diverged from one another in the earliest stages of organic life, and had separated widely before any of the types we now know had come into existence. ”

“ Von Baer lived in the days when the Development Hypothesis was mentioned only to be ridiculed, and he joined in the ridicule. What he conceived to be the meaning of these groupings of organisms and these relations among their embryological histories, is not obvious. The only alternative to the hypothesis of Evolution is the hypothesis of Special Creation ”

“ The young artillery officer is first taught that a cannon-shot describes a curve treated as a parabola, though literally part of an extremely eccentric ellipse not distinguishable from a parabola. Presently he learns that atmospheric resistance, causing a continual decrease of velocity, entails a deviation from that theoretical path which is calculated on the supposition that the velocity is uniform; and this incorrectness he has to allow for. Then, further, there comes the lateral deviation due to wind, which may be appreciable if the wind is strong and the range great. ”

“ Much more striking are examples furnished by creatures whose modes of existence require that they shall have enormous fertility—require that the generative system shall be very large. Ordinarily the organs devoted to maintenance of the race develop later than the organs devoted to maintenance of the individual. ”

“ From the principle of economy, it seems inferable that decrease and disappearance of organs which were useful in ancestral types but have ceased to be useful, should take place uniformly; but they do not. ”

“ On carrying this idea with us, we shall see that if from some type of organism there is evolved a type in which enlargement of a certain part is needed to meet increased functions, the greater size of this part will begin to show itself during early stages of unfolding. ”

“ It is impossible to express the totality of any natural phenomenon in a single proposition. To the primary statement expressing that which is most dominant have to be added secondary statements qualifying it. ”

“ It seems to be a general fact that a structure which in metamorphosis disappears completely may easily be omitted altogether in embryonic development, while one which is modified into something else continues to pass more or less through its original larval condition. ”

“ A disused organ which entails evil because construction of it involves needless cost, may entail further evil by being in the way. This, it seems to me, is the reason why the fore-limbs of snakes have disappeared from their embryos. ”

“ In the evolution of these embryos with which there is not laid up a large amount of food-yolk there occurs at the outset a striking omission of this kind. When, by successive fissions, the fertilized cell has given rise to a cluster of cells constituting a hollow sphere, known as a blastula, the next change under its original form is the introversion of one side, so as to produce two layers in place of one. ”

“ To these creatures, imbedded in nutritive matters, self-maintenance cost nothing, and the structures devoted to it are relatively of less importance than the structures devoted to race-maintenance, which, to make up for the small chance any one germ has of getting into a fit habitat, have to produce immense numbers of germs. ”

“ Of two cognate races which diverged in the remote past, the one may have had descendants that have remained tolerably constant in their habits, while the other may have had descendants that have passed through widely-aberrant modes of life; and yet some of these last may have eventually taken to modes of life like those of the other races derived from the same stock. And if the metamorphoses of embryos indicate, in a general way, the changes of structure undergone by ancestors ”

“ Yet another cause operates, and perhaps cooperates. Embryonic survival of an organ which has become functionless, may readily happen if, during subsequent stages of development, parts of it are utilized as parts of other organs. ”

“ Often a kindred passing over of stages at later periods of development may be observed. In certain of the Mollusca, as the Patella chiton, the egg gives origin to a free-swimming larva known as a trochosphere, from which presently comes the ordinary molluscous organization. In the highest division of the Molluscs, however, the Cephalopods, no trochosphere is formed. The nutritive matter laid up in the egg is used in building up the young animal without any indication of an ancestral larva. ”

“ If this piece of work is not promptly commenced and pushed on fast, it will not be completed when the rest of the house is ready: workmen and tools will still block it up at a time when it should be available. Similarly among the parts of an unfolding embryo, those in which there is a great deal of constructive work must early take such shape as will allow of this. ”

“ For the successive changes in modes of life pursued by successive ancestral races, can have had no regularity of sequence. In some cases they must have been more numerous than in others; in some cases they must have been greater in degree than in others ”

“ Petrels are the most aërial and oceanic of birds, but in the quiet sounds of Tierra del Fuego, the Puffinuria berardi, in its general habits, in its astonishing power of diving, its manner of swimming, and of flying when unwillingly it takes flight, would be mistaken by any one for an auk or grebe ”

“ Allied to these cases, are the cases of what has been called retrograde development. Many parasitic creatures and creatures which, after leading active lives for a time, become fixed, lose, in their adult states, the limbs and senses they had when young. ”

“ When the long-bodied lizard out of which the ophidian type evolved, crept through stiff herbage, and moved its head from side to side to find openings, there resulted alternate bends of its body, which were the beginnings of lateral undulations ”