LAW OF DEVELOPMENT KNOWN AS VON BAER’S LAW. 49 
of the adult animal with one another. I repeat, the constancy 
of development in the same species proves this point, as do the 
small but constant differeuces between the embryonic phases of 
slightly different but distinct species. 
Granting that embryonic rudiments do vary, of which I do 
not think there can be any doubt, then it would appear that 
the variations must be selected, not with regard to their 
intrinsic! merits at the moment as is the case with variations 
in functional organs, but with regard to the effect of their de- 
scendent or correlated variations in the adult. So it comes 
about that the embryonic rudiments in one group of animals, 
though resembling generally those of another group of the 
same class or phylum (just as the functional adult organs 
resemble one another generally), yet differ from them in minor 
points, so that the group has its own individual character with 
regard to that particular rudimentary organ, just as it has its 
own individual character with regard to any adult functional 
organ. 
The conclusion here reached is that, whereas larval develop- 
ment must retain traces (it may be very faint) of ancestral 
stages of structure because they are built out of ancestral 
stages, embryonic development need not necessarily do so, 
and very often does not; that embryonic development, in so far 
as it is a record at all, is a record of structural features of pre- 
vious larval stages. Characters which disappear during free 
life disappear also in the embryo, but characters which though 
lost by the adult are retained in the larva may ultimately be 
absorbed into the embryonic phase and leave their traces in 
embryonic development. 
[Throughout the above discussion I have, to avoid complica- 
tion, treated all embryonic organs to be functionless; but it 
1 By intrinsic merits at the moment, I mean the effect on the organism as a 
whole at any particular moment. A variation in a rudimentary functionless 
organ of an embryo can have no effect upon the welfare of the embryo 
(excluding secondary effects—if any—of interfering with functional organs, 
e. g. blood-vessels) ; its utility can only be judged when the free state is 
reached. 
vou. 36, PART 1.—NEW SER. D 
