LAW OF DEVELOPMENT KNOWN AS VON BAER’S LAW. 39 
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, | 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 
