LAW OF DEVELOPMENT KNOWN AS VON BAEE's LAW. 79 



two species so closely similar that I was long in doubt whether 

 they were distinct species, viz. Peripatus Capensis and 

 Balfouri, it would be useless to look for embryonic differ- 

 ences : yet I can distinguish a fowl and a duck embryo on 

 the second day by the inspection of a single transverse section 

 through the trunk, and it was the embryonic differences between 

 the Peripatuses which led me to establish without hesitation the 

 two separate species. But it is not necessary to emphasise 

 further these embryonic differences; every embryologist knows 

 that they exist and could bring forward innumerable instances 

 of them. I need only say with regard to them that a species 

 is distinct and distinguishable from its allies from 

 the very earliest stages all through the develop- 

 ment, although these embryonic differences do not 

 necessarily implicate the same organs as do the 

 adult differences. 



If I have laid great stress — some may think undue stress — 

 upon the inadequacy of v. Baer's law, I have done so be- 

 cause of the importance which is at the present day attached 

 to this law by teachers of zoology. In support of this, I may 

 quote the words of three of the greatest teachers of zoology of 

 this or of any other age — words which show that they at any 

 rate considered that the law correctly represented the facts. 



Darwin, in the 'Origin' (p. 387, 6th ed.), says: — "So 

 again it has been shown that generally the embryos of the 

 most distinct species belonging to the same class are closely 

 similar, but become, when fully developed, widely dissimilar. 

 A better proof of this latter fact cannot be given than the 

 statement by v. Baer that the embryos of Mammalia, of 

 birds, of lizards, and snakes, probably also of Chelonia, are in 

 their earliest states exceedingly like one another, both as a 

 whole and in the mode of development of their parts ; so much 

 so, in fact, that we can often distinguish the embryos only by 

 their size. In my possession are two little embryos in spirit, 

 whose names I have omitted to attach, and at present I am 

 quite unable to say to what class they belong. They may be 

 lizards or small birds, or very young Mammalia, so complete 



