88 ADAM SEDGWICg. 



bryouic 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 aifected, 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 



