46 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 larvee 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 larve will be the more complete the 
more completely the ancestral habits of life are retained by 
them. There is, then, in larve a tendency to the inheritance of 
variations at corresponding periods, and in this respect larve 
differ 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 uo 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 larve after they have been lost by the adult is 
