304 THE THEORIES OF EVOLUTION 



"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 varia- 

 tions of an original and fixed number of elements con- 

 stitute their only differences. A part large in one 

 animal may be small in another, or vice versa ; or com- 

 plex in one and simple in another. . . . 



"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 a species with three 

 toes would rank in every other respect between species 

 with one and four toes. ) 



"3. Parallelism. — This states that all organisms in 

 their embryonic and later growth pass through stages 

 which recapitulate the successive permanent conditions 

 of their ancestry. . . ." 



"4. Teleology. — This is the law of fitness of struc- 

 tures for their special uses, and it expresses broadly 

 the general adaptations of an animal to its home and 

 habits." 4 



Eimer's and Cope's theories are frankly Lamarck- 

 ian in tendency; both also have recourse to facts re- 

 vealed by observation and experiment, the action of the 

 environment or the conscious reaction against it, to ac- 

 count for the definite direction followed by evolution. 



4 Primary Factors of Organic Evolution, pp. 19-20. 



