620 TRANSACTIONS OP SECTION D. 



To answer this question we have to be sure of what we mean when we speak of 

 the Darwinian theory. I think that we mean at least two things. (1) That the 

 assemblage of animal forms as we now see them, with all their diversities of form, 

 habit, and structure, is directly descended from a precedent and somewhat 

 different assemblage, and these in turn from a precedent and more different 

 assemblage, and so on down to remote periods of geological time. Further, that 

 throughout all these periods inheritance combined with changeability of structure 

 have been the factors operative in producing the differences between the succes- 

 sive assemblages. (2) That the modifications of form which this theory of evolu- 

 tion implies have been rejected or preserved and accumulated by the action of 

 natural selection. 



As regards the first of these propositions, I think there can be no doubt that 

 morphology has done great service in establishing our belief on a secure basis. 

 The transmutation of animal forms in past time cannot be proved directly ; it can 

 only be shown that, as a theory, it has a much higher degree of probability than any 

 other that can be brought forward, and in order to establish the highest possible 

 degree of probability.it was necessary to demonstrate that all anatomical, embryo- 

 logical, and palasontological facts were consistent with it. We are apt to forget, 

 nowadays, that there is no a j'i'iori reason for regarding the resemblances and 

 differences that we observe in organic forms as something different in kind from the 

 analogous series of resemblances and differences that obtain in inanimate objects. 

 This was clearly pointed out by Fleeming Jenkin in a very able and much- 

 referred to article in the ' North British Review ' for June 1867, and his argument 

 from the a priori standpoint has as much force to-day as when it was written 

 forty-three years ago. But it has lost almost all its force through the arguments 

 a posteriori supplied by morphological science. Our belief in the transmutation 

 of animal organisation in past time is founded very largely upon our minute and 

 intimate knowledge of the manifold relations of structural form that obtain 

 among adult animals ; on our precise knowledge of the steps by which these adult 

 relations are established during the development of different kinds of animals ; 

 on our constantly increasing knowledge of the succession of animal forms in past 

 time ; and, generally, on the conviction that all the diverse forms of tissues, 

 organs, and entire animals are but the expression of an infinite number of varia- 

 tions of a single theme, that theme being cell division, multiplication, and 

 differentiation. This conviction grew but slowly in men's minds. It was opposed 

 to the cherished beliefs of centuries, and morphology rendered a necessary service 

 when it spent all those years which have been described as 'years in the wilder- 

 ness ' in accumulating such a mass of circumstantial evidence in favour of an 

 evolutionary explanation of the order of animate, nature as to place the doctrine 

 of descent with modification on a secure foundation of fact. I do not believe 

 that this foundation could have been so securely laid in any other way, and I 

 hold that zoologists were actuated by a sound instinct in working so largely on 

 morphological lines for forty j'ears after Darwin wrote. For there was a large 

 mass of fact and theory to be remodelled and brought into harmony with the new 

 ideas, and a still larger vein of undiscovered fact to explore. The matter was 

 difficult and the pace could not be forced. Morphology, therefore, deserves the 

 credit of having done well in the past : the question remains, What can it do in 

 the future ? 



It is evident, I think, that it cannot do much in the way of adding new truths 

 and general principles to zoological science, nor even much more that is useful in 

 the verification of established principles, without enlarging its scope and methods. 

 Hitherto — or, at any rate, until very recently — it has accepted certain guiding 

 principles on faith, and, without inquiring too closely into their validity, has 

 occupied itself with showing that, on the assumption that these principles are 

 true, the phenomena of animal structure, development, and succession receive 

 a reasonable explanation. 



We have seen that the fundamental principles relied upon during the last fifty 

 years have been inheritance and variation. In every inference drawn from the 

 comparison of one kind of animal structure with another, the morphologist 

 founds himself on the assumption that different degrees of similitude correspond 

 more or less closely to degrees of blood-relationship, and to-day there are probably 

 few persons who doubt that this assumption is valid. But we must not forget 

 that, before the publication of the ' Origin of Species,' it was rejected by the 

 most influential zoologists as an idle speculation, and that it is imperilled by 



