CHAPTER XVIII 

 SUMMAKY 



OUK survey of the embryology of the Invertebrata is now completed, 

 but, before closing the volume, it seems desirable to pause for a 

 brief space and reflect, so that, if there are any general principles 

 to be deduced from the study of the series of life -histories which 

 have been described, they may not escape us. 



The first and most far-reaching conclusion which we may draw, 

 is that, in general, the larval phase of development represents a 

 former condition of the adults of the stock to which it belongs. 

 This, in substance, is of course the recapitulatory theory of develop- 

 ment, the famous biogenetic law of Haeckel. In these days this 

 law is regarded with disfavour by many zoologists, so that to rank 

 oneself as a supporter of it is to be regarded as out-of-date. The 

 newest theory is, however, not necessarily the truest ; and this 

 we may certainly say, that if there has been evolutionary change 

 at all, which no one seriously doubts, if every species of animal has 

 not been created, adapted to the conditions in which we now find it, 

 then, nothing can lie more certain than that the parasitic members 

 of the great natural groups are descended from ancestors which con- 

 formed in their structure to the normal type of the group in question. 



Now, in the life -histories of these parasites, the larva, in almost 

 every case, shows an unmistakable resemblance to the normal type 

 of adult in the group. It appears to us that we have in such animals 

 a critical case by which we can test the truth of the recapitulatory 

 theory ; since, so far as human intelligence goes, the ancestry of such 

 creatures is known. Does any naturalist seriously doubt that the 

 ancestors of Adheres ambloplitis, described in Chapter VIII., were 

 once ordinary Copepoda ? As Metschnikoff has said, parasites are 

 really the latest products of evolution. 



If, however, the recapitulation of ancestral .structure turns out 

 to be the primary explanation of developmental history in cases 

 where the ancestry is known, surely we have the right to assume that 

 the same type of explanation is valid where the ancestry is other- 

 wise unknown, and to conclude that in general the larval phase has a 

 recapitulatory significance. 



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