364 COOK 



The exclusion of the domesticated plants and animals from 

 use as illustrations of the true methods of evolution may appear 

 to withdraw the subject from the consideration of all who do 

 not have intimate acquaintance with' species in nature. There 

 remains, however, an excellent and very familiar example of 

 evolutionary conditions, that of man himself. The genus Homo 

 has achieved in a relatively brief period a wide divergence 

 from its simian relatives. This progress in development has 

 been coincident with the achievement of a world-wide distribu- 

 tion and with free interbreeding throughout the area of distribu- 

 tion, except as hindered by geographical barriers. Moreover, 

 a further close analogy is to be found in the development of the 

 human individual personality by a complex network of contacts 

 with other members of a social group. Without such social 

 contacts the intellectual development was limited to automatic 

 instincts ; with socialization new lines of evolution became pos- 

 sible, just as conjugation opened the road to the development of 

 compound organisms, and the further various stages of advance 

 in prolonged conjugation made possible higher and higher types 

 of cellular structures. 



LONGITUDINAL AND TRANSVERSE SECTIONS OF SPECIES. 



Longitudinal sections of species show differences along lines 

 of descent. They include what are commonly called life-his- 

 tories, based on studies of the progressive changes of form and 

 of methods of existence by which individual organisms follow 

 each other in lines of descent. 



Transverse sections of species show differences and relations 

 between lines of descent, that is, the internal bionomy of the 

 species. The objects of study are not the methods of develop- 

 ment or the physiology of individuals as such, but the nature and 

 relations of the different kinds of individuals which exist in the 

 species. The individuals of a species which are alive at any 

 one time may be thought of as affording a cross-section or end 

 view of the network of descent. 



Some of the facts of the constitution of species can be under- 

 stood best from longitudinal sections, some from cross-sections, 

 and many can be best thought of by keeping both aspects of the 

 network in mind. 



