658 INVEETEBEATA chap. 



Speaking of the geological record, Darwin wrote, in 1859, that of 

 the book of life we only possess the last volume. This conclusion, so 

 far as Invertebrates are concerned, has been amply confirmed by the 

 remarkable discovery by Walcott (1912), of a richly differentiated 

 and exquisitely preserved fauna in the Mid-Cambrian rocks of North 

 America. If therefore the relationship to one another, not only of 

 the various phyla of the animal kingdom, but even of the classes 

 TOthin the phyla, is ever to be elucidated, it can only be done by 

 Comparative Embryology. Palaeontology begins too late to undertake 

 the task. 



From all the discussion, however, which has just been completed, 

 of the secondary factors which modify the recapitulatory record in 

 both embryonic and larval hfe, it will be clear that only an Embryology 

 based on an enormous collection of facts, and a careful comparison of 

 one type with another, can hope to discriminate between primary and 

 secondary factors in development, and to elucidate ancestral history. 



It is because too many zoologists have based far-reaching theories 

 on the hfe-history of some one type, that recapitulatory embryology 

 has fallen into bad repute. But the remedy for the abuse of little 

 light is more light, and already, by the steady accumulation of 

 embryological data and the improvement of methods, some of the 

 controversies which vexed our fathers in the 'eighties are in affair 

 way to be settled. 



The late Professor Welldon pointed out that it would be of great 

 interest and importance to find out how far the most recent evolu- 

 tionary changes, such as those which made the difference between 

 genera within a family, and between families within a tribe, are 

 represented in the life-history. Here is a field for research of great 

 importance, in which very httle work has as yet been done. These 

 latest stages of the life-history should be the freest from disturbance 

 by secondary factors, but it is precisely over the interpretation of the 

 earliest stages in the life-history, which represent the oldest pages in 

 the ancestral record, and which one would expect to 'be most blurred, 

 that the greatest disputes have arisen. 



There is a school of embryologists, led by Driesch, who decry the 

 value of the recapitulatory interpretation altogether, and who insist 

 that the developmental processes of every animal should be referred 

 back to causes existing in its egg, and that no hypothetical ancestry 

 should be called in to aid in the explanation. No doubt it is true, 

 theoretically, that the stages in the development of an animal form a 

 continuous series, and that the causes for the production of each 

 stage are to be found in the preceding one. But even if this causal 

 chain were completely elucidated, it would leave entirely unexplained 

 the marvellous resemblances between the larvae of some species and 

 the adult stages of others, or between the larvae of widely different 

 groups ; and it is precisely phenomena of this kind that Comparative 

 Embryology seeks to account for. 



In this search Comparative Embryology has been very greatly 



