XVI] THE PROBLEM OF PHYLOGENY 1025 



even from a tooth, but by the discovery of forms so intermediate 

 in their general structure as to indicate an organisation and, ipso 

 facto, a mode of life, intermediate between the terrestrial and the 

 Cetacean form. There is no vahd syllogism to the effect that A 

 has a flat curved scapula like a seal's, and B has a fl.a,t curved 

 scapula like a seal's : and therefore A and B are related to the seals 

 and to each other; it is merely a flagrant case of an "undistributed 

 middle." But there is validity in an argument that B shews in 

 its general structure, extending over this bone and that bone, 

 resemblances both to A and to the seals : and that therefore he may 

 be presumed to be related to both, in his hereditary habits of life 

 and in actual kinship by blood. It is cognate to this argument that 

 (as every palaeontologist knows) we find clues to affinity more 

 easily, that is to say with less confusion and perplexity, in certain 

 structures than in others. The deep-seated rhythms of growth 

 which, as I venture to think, are the chief basis of morphological 

 heredity, bring about similarities of form which endure in the 

 absence of conflicting forces ; but a new system of forces, introduced 

 by altered environment and habits, impinging on those particular 

 parts of the fabric which he within this particular field of force, will 

 assuredly not be long of manifesting itself in notable and inevitable 

 modifications of form. And if this be really so, it will further 

 imply that modifications of form will tend to manifest themselves, 

 not so much in small and isolated phenomena, in this part of the 

 fabric or in that, in a scapula for instance or a humerus : but rather 

 in some slow, general, and more or less uniform or graded modifica- 

 tion, spread over a number of correlated parts, and at times 

 extending over the whole, or over great portions, of the body. 

 Whether any such general tendency to widespread and correlated 

 transformation exists, we shall attempt to discuss in the following 

 chapter. 



