348 THE ZOOLOGIST. 



study of form. And in treating of form they have all, since the 

 acceptance of the doctrine of descent with modification, made use of 

 the same guiding principle — namely, that likeness of form is the 

 index to blood-relationship. It was the introduction of this principle 

 that revolutionized the methods of morphology fifty years ago, and 

 stimulated that vast output of morphological work which some 

 persons, erroneously as I think, regard as a departure from the line 

 of progress indicated by Darwin. 



We may now ask, What has morphology done for the advance- 

 ment of zoological science since the publication of the ' Origin of 

 Species ' ? We need not stop to inquire what facts it has accu- 

 mulated : it is sufficiently obvious that it has added enormously to 

 our stock of concrete knowledge. We have rather to ask, What great 

 general principles has it established on so secure a basis that they 

 meet with universal acceptance at the hands of competent zoologists ? 



It has doubtless been the object of morphology during the past 

 half-century to illustrate and confirm the Darwinian theory. How 

 far has it been successful ? 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 in- 

 heritance combined with changeability of structure have been the 

 factors operative in producing the differences between the successive 

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

 of evolution 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, embryological, and palaeontological facts were con- 

 sistent with it. We are apt to forget, nowadays, that there is no 

 a priori 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 

 Eeview ' for June, 1867, and his argument from the a priori stand- 

 point 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 organization 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 



