454 COSMIC PHILOSOPHY. [pt. ii, 



those persons take a quite similar position, who pretend 

 that God created separately the horse, ass, zebra, and 

 quagga, having previously created a beast enough like all 

 of them to be their common grandfather. Indeed, so powerful 

 is this argument from classification that it has always seemed 

 to me sufficient by itself to decide the case in favour of the 

 theory of derivation. In my own case, the facts presented 

 in Prof. Agassiz's " Essay on Classification " went far toward 

 producing conviction before the publication of Mr. Darwin's 

 work on the " Origin of Species," where the significance of 

 such facts is clearly pointed out and strongly insisted upon. 



II. An equally powerful argument is furnished by the 

 embryonic development of organisms. As Von Baer long 

 ago pointed out, the germs of all animals are at the outset 

 exactly like each other ; but in the process of development 

 each germ acquires first the differential characteristics of the 

 sub-kingdom to which it belongs, then successively the 

 characteristics of its class, order, family, genus, species, and 

 race. For example the germ-cell of a man is not only' in- 

 distinguishable from the germ- cell of a dog, a chicken, or a 

 tortoise, but it is like the adult form of an amceba or a 

 protococcus, which are nothing but simple cells. Four weeks 

 after conception, the embryos of the man and the dog can 

 hardly be distinguished from each other, but have become 

 perceptibly different from the corresponding embryos of the 

 chicken and tortoise. At eight weeks a few points of differ- 

 erence between the dog and the man become perceptible; 

 the tail is shorter in the human embryo, and the cerebrum 

 and cerebellum have become larger, relatively to the corpora 

 quadrigemina, than in the embryo of the dog; but these 

 differences are less striking than those which separate the 

 two mammals on the one hand from the reptile and bird on 

 the other. At a later stage the human embryo becomes still 

 more unlike that of the dog, acquiring characteristics 

 peculiar to the order of primates to which man belongs 



