48 ADAM SEDGWICK. 
bryonic only, the conditions of its existence are totally altered. 
Its disappearance is no longer a matter of importance to the 
organism, because the embryo being protected from the struggle 
for existence the presence of rudimentary functionless organs 
is unimportant to it. They therefore persist, and it is this 
persistence which has given rise to von Baer’s law. But von 
Baer’s law is imperfect, because it omits to take cognisance of 
the fact that embryonic features are no more constant than are 
the adult characters; that indeed they vary with the adult 
characters, so that no adult character is changed without some 
precedent alteration of all the previous embryonic phases. ‘The 
embryonic life is a connected whole, and it is impossible that 
an isolated alteration of one particular stage can have taken 
place. All variations must run through the whole develop- 
ment; they may come out strongly at one particular stage, but 
they must have been led up to and followed by variations in 
all other stages. 
Embryonic variations are not for the most part acted upon 
by natural selection, because they concern rudimentary organs 
only ; but when free life is reached, and the organs become 
functional, the same variations continued (for continue they 
must) are put to the test, and the organism stands or falls by 
them. ‘The constancy of development in the same species 
proves this point ; for if the embryonic stage could vary without 
the free stages being at all affected, then, as natural selection 
does not act upon rudimentary embryonic organs, the em- 
bryonic organs would run riot, and we should expect to find 
the greatest diversity in embryonic development of the same 
species, and this we do not find; and this applies not only to 
organs which persist into the adult, but also to organs which 
disappear before the adult stage is reached. These purely 
embryonic structures must have some nexus with structures 
which succeed them in development, and a variation in them 
must be accompanied by variations in these later appearing 
persistent organs. In fact, it seems to me most important to 
remember that the various stages in the development of an 
animal are just as much correlated as are the different organs 
