CH. xxxix. EMBRYOLOGY. 439 



For example, if you could watch the beginning of these four 

 animals, there would be a certain time during which you 

 could see no difference in their form. Then after a while 

 the fish would start off on a road of its own, but still the 

 other three would go on all alike. Then, when they had 

 grown a little bigger, the lizard would branch off, and only 

 the bird and the ox would continue to have the same form, 

 until lastly the bird would take on its own peculiar shape, 

 and the ox would go on alone, having passed through the 

 same stages as the fish, the reptile, and the bird, before it 

 began to shape itself like a mammal. You must notice 

 carefully that this does not mean that the beginning of an 

 ox is at any time like a full-grown fish, which is a mistake 

 that people often make; but only that there is a time 

 when the minute embryos of these animals are almost in- 

 distinguishable the one from the other. 



You will see, if you consider for a moment, that the dis- 

 covery of this curious fact gave naturalists a new and much 

 more perfect way of classifying animals ; for they could 

 actually read the history of an animal by watching it in the 

 earlier stages of its growth and seeing at what point it 

 branched off and put on special peculiarities of its own ; and 

 in some of the lower and more obscure animals several mis- 

 takes of classification were corrected by this means. There 

 was also another very important question settled by Von 

 Baer's law. It proved that St.-Hilaire was certainly right in 

 saying that animals are formed on one plan, having special 

 parts altered to suit their wants, for here in the embryo those 

 parts can be seen actually developing differently in different 

 animals out of the same beginnings. The study of embry- 

 ology has been carried to great perfection since Von Baer 

 published his ' History of the Development of Animals ' in 



