86 ADAM SEDGWICK. 



structure and retaining some ancestral habit of life. But of 

 course these larval stages are liable to vary and are subject 

 to the natural selection engendered by the struggle for life. 

 So they may themselves become modified and the ancestral 

 habits and structure which they have inherited may also become 

 modified. It thus becomes apparent that larvae will often retain 

 traces more or less complete of ancestral stages of structure, 

 and that they will do this in virtue of the operation of the 

 force of heredity and of natural selection. And the retention 

 of ancestral features by the larvae will be the more complete the 

 more completely the ancestral habits of life are retained by 

 them. There is, then, in larvae a tendency to the inheritance of 

 variations at corresponding periods, and in this respect larvae 

 diff'er from embryos. 



To sum up, I would maintain that ancestral stages of struc- 

 ture are only retained in so far as they are useful to the free- 

 growing organism, i. e. to the larva in its free development. 

 Or, to put the matter in another and more recondite form, 

 modifications appearing in and affecting the adult structures 

 will similarly affect the same structures all through the 

 development of the offspring unless the old structural arrange- 

 ments are called into being in the development of the offspring 

 by the application of the old stimulus, viz. the same external 

 conditions of life. 



In embryos, on the other hand, the organs are for the most 

 part functionless, and there appears to be no reason for the 

 retention of ancestral conditions of structure. On the con- 

 trary, as I have shown above, most organs when modified in 

 the free-living state are similarly modified in the embryo. 

 And, as I have already insisted, this is what we should expect 

 when we remember that embryonic development is the pre- 

 paration of the free form in the most perfect state and at the 

 least expense. How is it, then, that we do get in embryos in 

 certain cases a most remarkable preservation of ancestral 

 organs and conditions of structure which have been lost in the 

 adult ? I think it can be shown that the retention of ancestral 

 organs by the larvae after they have been lost by the adult is 



