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int: 



LTURALIST [Vol. XLIII 



acters save size and reductive maturity from the young 

 of the other kelps which have been studied. But Ren- 

 frewia, juvenile or adult, is not one of the ancestors of 

 these higher kelps. It is only a simpler form which we 

 take to have been left behind in the evolution of the 

 kelps. Our actual knowledge of their ancestors is al- 

 most nothing. But if we were to reconstruct a general- 

 ized common ancestor for the kelps, by projecting back- 

 ward, from the different tribes, lines indicating their ap- 

 parent course of evolution, until they converged and met, 

 we should have to conceive a plant very similar in all 

 respects to Renfrewia. 



What then is to be said concerning structures which 

 do not recapitulate adult but only embryonic conditions? 

 In the toothless animals, the whale and the bird, the de- 

 velopment of teeth in the jaw is entirely unnecessary, as 

 has been pointed out in considering His's idea. It may 

 even be said to hinder the attainment of the adult con- 

 dition. The same is true of the mammalian gill-slits and 

 of most of the structures which have in the past attracted 

 attention in connection with the recapitulation theory. 

 As the ancestral period, when such structures were fully 

 developed in the adult, becomes more and more remote, 

 the tendency to inherit them becomes less and less, be- 

 cause of the cumulative impulses given to the heritage 

 by the nearer ancestors. Consequently, they are succes- 

 sively less and less developed. Any gradual loss of in- 

 herited structures can, in the nature of the case, take 

 place only from the mature condition backward towards 

 the beginning of the life cycle ; otherwise we should have 

 adult structures with no ontogenetic history. Therefore 

 we can understand why it is that in many cases only the 

 embryonic stages of ancestral organs persist in the on- 

 togeny. 3 



» The cutting off of end stages in the development of organs has given 

 rise to the idea that the adult stages are " pushed back into the embryo." 

 Such a misconception easily arose from the loose language in which the 

 facts have often been expressed. Conklin ( '05) has rightly pointed out 



