296 THE STRUCTURAL EVIDEXCK OF EYOLUTIOi^. 



mold. It is one of the most wonderful phenomena of nature, 

 that such complex organisms, consisting of so many parts, should 

 be repeated from age to age, and from generation to generation, 

 with such surprising fidelity and precision. This fact is the first 

 that strikes the student of these sciences. The first impression 

 of the ordinary person would be, that these things must continue 

 unchanged. When I began to study zoology and botany, I was 

 surprised to find there was a science of which I had no concep- 

 tion, and that was this remarkable reproduction of types one 

 after another in continued succession. After a man has had 

 this idea thoroughly assimilated by his honest and conscien- 

 tious studies, he will be again struck with another class of 

 facts. He will find, not unfrequently, that this doctrine does 

 not apply. He will find a series of facts which show that many 

 individuals fail to coincide with their fellows precisely, the most 

 remarkable variations and the most remarkable half-way atti- 

 tudes and double-sided aspects occurring ; and he will come to 

 the conclusion, sooner or later, that like does not produce like in 

 some animals with the same precision and fidelity with which it 

 is accomplished in other animals. So that we have these two 

 classes of facts — the one relating to, and expressing, the law of 

 heredity ; the other which expresses the law of variation. I 

 should not like to say which class of facts is the most numerously 

 presented to the student. In the present fauna we find many 

 groups of species and varieties ; but exactly how many species we 

 have, how many genera and families we have, we can not defi- 

 nitely state. The more precise and exact a person is in his defini- 

 tion and in his analysis, the more definite his science becomes, 

 and the more precise and scientific his work. Biology is a science 

 of analvsis of forms. What the scales are to the chemist and the 

 physicist, the rule and measure are to the biologist. It is a ques- 

 tion of dimension, a question of length and breath and thickness, 

 a question of curves, a question of crooked shapes or simple 

 shapes — rarely simple shapes, mostly crooked, generally bilateral. 

 It requires that one should have a mechanical eye, and should 

 have also something of an artistic eye, to appreciate these forms, 

 to measure them, and to be able to compare and weigh them. 



Now, when we come to arrange our shapes and our measure- 

 ments, we find, as I said before, a certain number of identities, 

 and a certain number of variations. This question of variation 

 is so common and so remarkable, that it becomes perfectly evi- 



