THE EVOLUTION OF ANIMAL FUNCTION 329 



a new species fitted to survive, should itself continue to exist 

 without further change, and mark time on the main line of 

 evolution. Such an expectation is clearly absurd. The most 

 that we can hope is that some species now existing have not 

 diverged very far from the main line along which more recent 

 species passed on the way to their present condition, and so 

 may in a rough way be regarded as earlier stages of a con- 

 tinuous process. This is a serious difficulty in the way of 

 tracing out the course of evolution of any functional capability. 

 For even if we were able to estimate with some approach to 

 accuracy the extent to which any given species had diverged 

 morphologically from the main line of evolution, we should 

 still be as far as ever from knowing whether the peculiarities 

 of functional capability which that species might present had or 

 had not been the subject of much recent modification. We have 

 no means of knowing a priori whether the course of functional 

 evolution is likely to follow in any way that of the morphological. 



May we not look for some help towards the solution of 

 the problem from the study of ontogenetic development ? 

 May we not make some use of the Biogenetic Law, that 

 ontogenetic development is a recapitulation of phylogenetic ? 

 The method is a tempting one, for it removes at a stroke 

 that fundamental difficulty which we have already mentioned — 

 the difficulty of finding a single set of conditions under which 

 cells taken from diverse animals could be compared. In 

 comparing cells from different stages of any one embryonic 

 development we should not meet those differences of concentra- 

 tion of tissue fluids which we have to face in the comparison 

 of cells taken from different animals. Unfortunately it has 

 become amply clear in recent years that the Biogenetic Law 

 is not a generalisation which can be applied in any such 

 crude and direct manner. The study of ontogeny from the 

 morphological side has shown often enough that the law breaks 

 down when brought into touch with the details of development, 

 seems in fact to reduce itself to the simple observation that the 

 early stages of development in different animals are more 

 closely similar than the later stages, and refuses any respon- 

 sibility beyond this fact. 



This failure of the Biogenetic Law need not mean that 

 ontogenetic study is to render no help to the understanding 

 of functional evolution. The very fact that the Biogenetic Law 



