750 VOLUTION . * int de discontinuity dans ses parties, ou qui, du moins, n en a < a a point ae w suite dfl quelque8 especes 3uTs S H en Vo m quelqueV t H en resulte que les espies SS terminent cliaqne rameau de la serie generale tiennent, an moms % n cSf Td autres especes voisines qni se nuancent avec elles VoUa ce que 1 etat bien connu des choses me met nontenant a Irtt d* demontrer. Je n ai besoin d aucnne hypothese m d aucune Cposftio^ pour cela : j en atteste tontes naturahstes observateurs. 3 In a remarkable essay l Meckel remarks "There is no good physiologist who has not been struck by the observation that the original form of all organisms is one and the same and that out of this one form, all, the lowest as well as the SU are developed in such a manner that the latter P^ through the permanent forms of the former as transitory stages. Aristotle, Hal S Harvey, Kielmeyer, Autenrieth, and many others have either made this observation incidentally, or, especially the latter, have dmwn particular attention to it, and drawn therefrom results of permanent importance for physiology." Meckel proceeds to exemplify the thesis, that the lower forms of animals represent stages in the course of the development of the higher, with a large series of : tions. A.fter comparing the Salamanders and the pei branchiate Urodda with the Tadpoles and the Frogs, and enunciating the law that the more highly any animal is organized the more quickly does it pass through the lower stages, Meckel goes on to say " From these lowest Vertebrata to the highest, and to the highest forms among these, the comparison between the embryonic condi tions of the higher animals and the adult states of the lower can be more completely and thoroughly instituted than if the survey is extended to the Invertebrata, inasmuch as the latter are in many respects constructed upon an altogether too dissimilar type ; indeed they often differ from one another far more than the lowest verte brate does from the highest mammal ; yet the following pages will show that the comparison may be also extended to them with interest. In fact, there is a period when, as Aristotle long ago said, the em bryo of the highest animal has the form of a mere worm, and, devoid of internal and external organization, is merely an almost struc tureless lump of polype-substance. Notwithstanding the origin oi organs, it still for a certain time, by reason of its want of an inter nal bony skeleton, remains worm and mollusk, and only later enters into the series of the Vertebrata, although traces of the vertebral column even in the earliest periods testify its claim to a place in that series." Op. cit. pp. 4, 5. If Meckel s proposition is so far qualified, that the com parison of adult with embryonic forms is restricted within the limits of one type of organization; and, if it is further recollected, that the resemblance between the permanent lower form and the embryonic stage of a higher form ir not special but general, it is in entire accordance witl modern embryology ; although there is no branch o biology which has grown so largely, and improved its methods so much since Meckel s time, as this. In its original form, the doctrine of " arrest of development," a? advocated by Geoffroy Saint-Hilaire and Serres, was no doubt an over-statement of the case. It is not true, for example, that a fish is a reptile arrested in its development or that a reptile was ever a fish; but it is true that the reptile embryo, at one stage of its development, is an organism which, if it had an independent existence, mus be classified among fishes; and all the organs of the reptilf pass, in the course of their development, through condition which are closely analogous to those which are permanen in some fishes. 4. That branch of biology which is termed Morpholog} is a commentary upon, and expansion of, the proposition that widely different animals or plants, and widely differen parts of animals or plants, are constructed upon the sam plan. From the rough comparison of the skeleton of a bin with that of a man by Belon, in the sixteenth century (t go no further back), down to the theory of the limbs an< 1 " Entwurf einer Darstelhrng der zvrischen dem Embryoznstanr der holicren Thiere und dem pernianenten der niederen stattflndende Parallele," Reytrage zur Vergleichenden Anatomie, Bd. ii. 1811. he theory of the skull at the present day ; or, from the rst demonstration of the homologies of the parts of a .ower by C. F. Wolff, to the present elaborate analysis f the floral organs, morphology exhibits a continual ad- ance towards the demonstration of a fundamental unity mon" the seeming diversities of living structures. And this lemonstration has been completed by the final establish ment of the cell theory, which involves the admission of a H-imitive conformity, not only of all the elementary struc- ures in animals and plants respectively, but of those in he one of these great divisions of living things with those n the other. No a priori difficulty can be said to stand n the way of evolution, when it can be shown that all mimals and all plants proceed by modes of development, vhich are similar in principle, from a fundamental proto plasmic material. 5. The innumerable cases of structures, which are rudi mentary and apparently useless, in species, the close allies of which possess well developed and functionally important lomologous structures, are readily intelligible on the theory of evolution, while it is hard to conceive their raison d etre on any other hypothesis. However, a cautious reasoner will probably rather explain such cases deductively from the doctrine of evolution, than endeavour to support the doctrine of evolution by them. For it is almost impossible to prove that any structure, however rudimentary, is useless that is to say, that it plays no part whatever in the economy; and, if it is in the slightest degree useful, there is no reason why, on the hypothesis of direct creation, it should not have been created. Nevertheless, double- edged as is the argument from rudimentary organs, there is probably none which has produced a greater effect in pro moting the general acceptance of the theory of evolution. 6. The older advocates of evolution sought for the causes of the process exclusively in the influence of varying conditions, such as climate and station, or hy bridization, upon living forms. Even Treviranus has got no further than this point. Lamarck introduced the con ception of the action of an animal on itself as a factor in producing modification. Starting from the well-known fact that the habitual use of a limb tends to develop the muscles of the limb, and to produce a greater and greater facility in using it, he made the general assumption that the effort of an animal to exert an organ in a given direc tion tends to develop the organ in that direction. But a little consideration showed that, though Lamarck had seized what, as far as it goes, is a true cause of modifica tion, it is a cause the actual effects of which are wholly inadequate to account for any considerable modification in animals, and which can have no influence at all in the vegetable world ; and probably nothing contributed so much to discredit evolution, in the early part of this century, as the floods of easy ridicule which were poured upon this part of Lamarck s speculation. The theory of natural selection, or survival of the fittest, was suggested by Wells in 1813, and further elaborated by Matthew in 1831. But the pregnant suggestions of these writers remained practically unnoticed and forgotten, until the theory was independently devised and promulgated by Darwin and Wallace in 1858, and the effect of its publication was immediate and profound. Those who were unwilling to accept evolution, without better grounds than such as are offered by Lamarck or tho author of that particularly unsatisfactory book, the Vestige* of the Natural History of the Creation, and who therefore preferred to suspend their judgment on the question, found; in the principle of selective breeding, pursued in all its ap plications with marvellous knowledge and skill by Mr Dar- win, a valid explanation of the occurrence of varieties and
races; and they saw clearly that, if the explanation would