CHAPTER XVII 

 EVOLUTION, VARIATION AND HEREDITY 



THE study of Biology is usually divided for convenience 

 into Botany and Zoology, and so far we have been concerned in the 

 main with the investigation of the facts of these subjects, particu- 

 larly as they relate to animals. The scientist is naturally concerned 

 with facts, for these of themselves are of great importance and often, 

 as in the study of medicine, of practical utility in conducting the 

 affairs of life. But while in the early stages it is necessary to devote 

 considerable time to acquiring a knowledge of these facts, and the 

 methods by which they can be verified and fresh ones discovered, 

 sooner or later one inevitably tries to put them in some sort of order, 

 to relate them to one another and generally to try and find the 

 causes that are working to produce them. This leads us into a field 

 of work where theories and facts are intermixed, often to such an 

 extent that it is impossible to draw a distinction between them. 

 When we pass on to examine certain of these theoretical points the 

 separation of Botany and Zoology, that is, the treatment of plants 

 and animals as beings that can be placed in isolated compartments, 

 is no longer convenient or possible. We may indeed draw our illus- 

 trations mainly from one or other kingdom, but they are only 

 examples and as a rule serve to illustrate principles that apply 

 directly or with only slight modification to both. Sometimes we 

 are able to see as the result of a number of observations that certain 

 definite conditions, which we term the cause or causes, are inevitably 

 followed by the appearance of definite phenomena which we call 

 the effect. When we can express this relationship in such a general 

 way that it includes a number of different phenomena, we say that 

 we have formulated a natural " law." It will be seen, however, that 

 the word " law" has a different meaning from that intended when 

 used in ordinary conversation. There is no implication that there 

 is a code of regulations drawn up, the breach of which is followed by 

 punishment, but simply that a certain effect is always produced as 

 the result of certain previous conditions. 



The most fundamental of the theories to be considered are those 

 that concern Evolution, or better, Organic Evolution. This idea of 



442 



