PRELIMINARY. 



THE structural relations of organisms may be ex- 

 pressed in the following canons : ^ 



1. Homology. — This means that organic beings are 

 composed of corresponding parts ; that the variations of 

 an original and fixed number of elements constitute 

 their only differences. A part large in one animal may 

 be small in another, or vice versa ; or complex in one 

 and simple in another. The analysis of animals with 

 skeletons, or Vertebrata, has yielded several hundred 

 original elements, out of which the twenty-eight thou- 

 sand included species are constructed. The study of 

 homologies is thus an extended one, and is far from 

 complete at the present day. 



2. Successional Relation. — This expresses the fact 

 that species naturally arrange themselves into series in 

 consequence of an order of excess and deficiency in 

 some feature or features. Thus species with three toes 

 naturally intervene between those with one and four 

 toes. So with the number of chambers of the heart, 

 of segments of the body, the skeleton, etc. There are 

 greater series and lesser or included series, and mis- 

 takes are easily made by taking the one for the other. 



1 Origin of the Fittest , p. 6. The laws here stated are as expressive of the 

 relations of plants as of animals. 



