102 THE DATA OF BIOLOGY. 



tated, works, by re-action, altered structure; and how in succeed- 

 ing generations, this altered structure may be made continu- 

 ally more marked by this altered function. Though 

 logically distinct, these two sub-divisions of biologic inquiry 

 cannot in practice be carried on apart. A speciality of struc- 

 ture which leads to an excess of function in any direction, 

 is, by the perpetual re-action of function, rendered ever more 

 decided. A speciality of function, by calling forth a corre- 

 sponding speciality of structure, produces an increasingly 

 efficient discharge of such function. Whichever of the two 

 initiates the change, there goes on between them an unceas- 

 ing action and re-action, producing in them co-ordinate 

 modifications. 



§ 41. The fourth great division of Biology, comprehend- 

 ing the phenomena of Genesis, may be conveniently separated 

 into three sub-divisions. 



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 or 

 sexual and asexual. An account of Sexual Multiplication in- 

 cludes the various methods 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 methods by which, from the same 

 fertilized germ or ovum, there are produced many organisms 

 that are partially or totally independent of each other. 



The second of these sub- divisions 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 sub-division is devoted to still more abstract 



