"microbes", that is, small living things; and that the microscopic organisms 

 which bring about decay arise in each case from others like themselves. 



Every single plant and every animal about which we have positive in- 

 formation has come from another organism of the same kind. Yet that all 

 life comes from life is one of those big ideas which we can never prove in 

 an absolute sense. Our knowledge is limited to what we have been able to 

 observe. In our general statements we reach out to other objects and events 

 of the same kjnd. In thus reaching out, we rely upon two important 

 assumptions: (1) We assume that things "of the same kind" are the same 

 in origin, structure, qualities, behavior, and so on. (2) We assume diat wt 

 can recognize things "of the same kind" when we come across them, with- 

 out always stopping to ask in exactly what ways and to exactly what extent 

 they are really "the same". 



Among so many different kinds of living things, it is conceivable that 

 they originated in different ways. Moreover, in our constant efforts to find 

 general rules or general ideas, we cannot help wondering what connections 

 there are between the various processes or events and the beginnings of new 

 individuals. What connection is there between modes of reproduction and 

 the conditions under which different species live? Are the methods the 

 same among plants as among animals? among land forms and water 

 forms ? Is there any connection between the length of life and the methods 

 of reproduction ? And why is it that some species produce very many tggs 

 or seeds, or many new individuals, whereas other species produce only one 

 or a very few offspring at one time ? 



Questions about the origin of new individuals may come in many cases 

 from idle curiosity. Yet answers often have important practical bearings. 

 It is important for us to produce large numbers of some kinds of plants and 

 animals, and it is important for us also to check the multiplication of otbe:s. 



342 



