THE RECAPITULATION THEORY. 33 



characteristic and essential mode of multiplication among 

 Metazoa ; that it occurs in all Metazoa ; and that when asexual 

 reproduction, as by budding, &c., occurs, this merely alternates 

 with the sexual process, which sooner or later becomes necessary. 



If the fundamental importance of sexual reproduction to the 

 welfare of the species be granted, and if it be further admitted 

 that Metazoa are descended from Protozoa, then we see that 

 there is a most powerful influence constraining every animal to 

 commence its life history in the unicellular condition, the only 

 condition in which the advantage of cross-fertilisation can be 

 obtained ; i.e. constraining every animal to begin its develop- 

 ment at its earliest ancestral stage, at the very bottom of its 

 genealogical tree. 



On this view the actual development of any animal is 

 strictly limited at both ends ; it must commence as an egg, 

 and it must end in the likeness of the parent. The problem of 

 recapitulation becomes thereby greatly narrowed ; all that 

 remains being to explain why the intermediate stages in the 

 actual development should repeat, more or less closely, the inter- 

 mediate stages of the ancestral history. Although narrowed in 

 this way, the problem still remains one of extreme difficulty, and 

 no final solution can yet be given of it. 



It is a consequence of the Theory of Natural Selection that 

 identity of structure involves community of descent ; a given 

 result can only be arrived at through a given sequence of 

 events. A negro and a white man have had common ancestors 

 in the past ; and it is through the long-continued action of 

 selection and environment that the two types have gradually 

 been evolved. You cannot turn a white man into a negro 

 merely by sending him to live in Africa : to create a negro the 

 whole ancestral history would have to be repeated, and it may 

 be that it is for the same reason that the embryo must repeat, 

 or recapitulate, its ancestral history in order to reach the adult 

 goal. 



Kleinenberg, in his ' Theory of the Development of Organs by 

 Substitution,' has suggested that each historic stage in the 

 evolution of an organ is necessary as a stimulus to the develop- 

 ment of the next succeeding stage, and that the reason for the 

 extraordinary persistence, in embryonic life, of organs which are 

 rudimentary and functionless in the adult, may be that the 



D 



