*' DEVELOPMENT AND PURPOSE CHAP. 



1 61- seeks to explain in the sense that he traces the whole 

 movement of organic life and the vast complexity of 

 organic forms to the operation of a few clearly established 

 empirical laws. The first of these laws is that all living 

 beings in the normal course of their life-history give rise 

 once or oftener to other living beings by separating off a 

 portion of their own tissue. The second is the equally 

 familiar fact that the new living beings, either directly, as 

 in the case of cell division, or after a number of cellular 

 generations, as in the case of sexual reproduction, come to 

 resemble their parent or parents in general type. The 

 third fact is that this resemblance is not absolute, but is 

 qualified by a certain degree of individual variation. The 

 fourth is that under some conditions such variations are 

 in turn perpetuated by heredity. The fifth is that of many 

 individuals born only a certain proportion among the 

 lower organic types only a very small proportion come 

 to maturity and so reproduce their species in turn. To 

 these may be added a sixth and last fact, that every living 

 being is born into an environment in which it has to main- 

 tain itself against dangers and provide itself with the 

 necessaries of life. 



These are for the most part very simple statements of 

 almost obvious fact. Yet in the hands of biological 

 science these very simple considerations go far to explain 

 the labyrinthine complexities of the actual development of 

 life on the earth. It is true that when we come to close 

 quarters certain of these statements raise questions of con- 

 troversy which are by no means so simple. What, for 

 example, are the nature and limits of that variation around 

 the parental type which manifestly forms the point of 

 departure for the entire process ? Are all variations quite 

 small and delicately graded so that there is always a con- 

 tinuity between any given type and any other that we 

 recognise as related but distinct? For a generation after 

 the publication of the Origin of Species it was the ambi- 

 tion of biologists to reduce all changes of form to variations 

 of this kind, and so exhibit evolution as a continuous 

 process. In later years, however, experiment seems clearly 

 to have shown that, explain them as we may, wider varia- 



