130 THE DATA OF BIOLOGY. 



action; and when it bequeaths more or less of its structural 

 peculiarity to descendants, among whom those which have it 

 most markedly are best able to thrive and propagate; there 

 arises a visibly modified type of structure, having a more or 

 less distinct function. In the correlative class of 



facts (by some asserted and by others denied), which come 

 under the category of reactions of function on structure as 

 exhibited in successions of organisms, are to be placed all 

 those modifications of structure which arise in races, when 

 changes of conditions entail changes in the balance of their 

 functions — when altered function externally necessitated, 

 produces altered structure, and continues doing this through 

 successive generations. 



§ 41. The fourth great division of Biology, comprehending 

 the phenomena of Genesis, may be conveniently separated 

 into three subdivisions. 



Under the first, comes a description of all the special 

 modes whereby the multiplication of organisms is carried on ; 

 which modes range themselves under the two chief heads of 

 sexual and asexual. An account of Sexual Multiplication 

 includes the various processes by which germs and ova are 

 fertilized, and by which, after fertilization, they are furnished 

 with the materials, and maintained in the conditions, needful 

 for their development. An account of Asexual Multiplica- 

 tion includes the various procesess by which, from the same 

 fertilized germ or ovum, there are produced many organisms 

 partially or totally independent of one another. 



The second of these subdivisions deals with the phenomena 

 of Genesis in the abstract. It takes for its subject-matter 

 such general questions as — What is the end subserved by the 

 union of sperm-cell and germ-cell? Why cannot all multi- 

 plication be carried on after the asexual method? What are 

 the laws of hereditary transmission? What are the causes 

 of variation? 



The third subdivision is devoted to still more abstract 



