26 INTRODUCTION. 



individual animals arise, not spontaneously, but by direct 

 descent from pre-existing animals, so also is it with species, with 

 families, and with larger groups of animals, and so also has it 

 been for all time ; that as the animals of succeeding generations 

 are related together, so also are those of successive geologic 

 periods; that all animals, living or that ever have lived, are 

 united together by blood relationship of varying nearness or 

 remoteness ; and that every animal now in existence has a 

 pedigree stretching back, not merely for ten or a hundred gene- 

 rations, but through all geologic time since life first commenced 

 on the earth. 



The study of development has in its turn revealed to us that 

 each animal bears the mark of its ancestry, and is compelled 

 to discover its parentage in its own development ; that the phases 

 through which an animal passes in its progress from the egg to 

 the adult are no accidental freaks, no mere matters of develop- 

 mental convenience, but represent more or less closely, in more 

 or less modified manner, the successive ancestral stages through 

 which the present condition has been acquired. 



Evolution tells us that each animal has had a pedigree in 

 the past. Embryology reveals to us this ancestry, because eveiy 

 animal in its own development repeats its history, climbs up its 

 own genealogical tree. 



This Recapitulation Theory, as it is termed, was obscurely 

 hinted at by the elder Agassiz, and suggested more directly in 

 the writings of Von Baer ; but it was first clearly enunciated by 

 Fritz Miiller in 1863, and has since that date formed the foun- 

 dation on which the explanation of the facts of embryology is 

 based. 



The fact that a frog commences its free existence as a tad- 

 pole, i.e. to all intents and purposes as a fish, is a very extra- 

 ordinary one, but it becomes at once intelligible if we interpret 

 it as meaning that frogs are descended from fish, and that every 

 frog is constrained to repeat or recapitulate its pedigree in the 

 course of its own individual development. 



Similarly, the long-tailed condition of the young crab at the 

 time of leaving the egg is to be viewed as an indication of the 

 descent of the short-tailed or brachyurous crustaceans from 

 macrurous ancestors; and the presence of gill clefts in the 

 young stages of chicks or rabbits, which when adult are totally 



