I INTEODUCTION 21 



of the early nineteenth century, for it is surely easier to conceive of 

 an All-embracing Intelligence, Whose myriad plans were realized in 

 the different species, rather than of millions of uncaused and un- 

 related intelhgences. Why, if the entelechy of a Strongylocentrotus 

 be entirely distinct from that of an Hchinus, should their products so 

 resemble one another? Has family resemblance in the animal 

 kingdom no meaning ? Our fathers attributed it to the Will of the 

 Creator. Darwin taught us to believe that it was due to descent 

 from a common stock. Driesch offers no explanation whatever, and 

 it seems to us that this final result is the reductio ad absurdum of his 

 whole system. Driesch's whole history has been that of the rebel 

 against accepted opinions, and in so far his intervention is healthy, 

 for nothing must be regarded as finally fixed, but pure reaction is 

 equally unjustified. 



Sedgwick's position (1909) is different. He formerly accepted the 

 biogenetic law, but as its application seemed to yield the most dis- 

 cordant results, he has been led to undertake a critical examination 

 of the assumptions on which it is based. He points out that it is 

 tacitly assumed that when a new feature appeared in the history of 

 the race this showed itself only in the adult condition, whilst the 

 previous adult condition was retained as a developmental stage ; that, 

 in a word, as evolution has proceeded the life-cycle of hving matter has 

 become more complex. Against such an assumption he points out that 

 a careful examination of the embryos of related species force us to the 

 conclusion that new features can appear at all stages of the life-history, 

 and that as all living matter known to us undergoes cyclical changes, 

 it is quite open to us to assume that this has always been the case since 

 the first appearance of living matter on the globe, and that therefore 

 the life-cycle has been modified but not extended. Neither of these 

 conclusions can be gainsaid a priori, and it is therefore time to take 

 stock of our data. 



It would lead us altogether too far to discuss the general proposi- 

 tion that zoological affinity means blood-relationship : this, we take it, 

 has been abundantly proved by the evidence, which Darwin has col- 

 lected, of the relationship which breeds, varieties, and species sustain to 

 one another ; if this be so it will be conceded that this zoological 

 affinity can be exhibited just as well by embryos as by adults, and 

 that, therefore, for the elucidation of biological affinity the study of 

 comparative embryology is necessary even if the biogenetic law be 

 baseless. 



On looking into the question of the vaUdity of this law, the first 

 question which presents itself for solution is the mutual relationship of 

 the embryonic and larval phases. On this subject Sedgwick himself 

 (1894) threw light some years ago, when he pointed out that the 

 embryonic phase is the remnant of a former larval phase, and that the 

 ancestral features which it exhibits are therefore features of a former 

 larva ; but these larval features, whether ancestral or not, consisted 

 of organs adapted to the larval mode of life. If, then, these features 



