174 WHAT IS LIFE ? 



tion. It is difficult to know in many cases where one 

 species begins or one ends. Nature abhors the rigid. 1 

 But many of the most marked types in this long 

 geological time a time which is infinite to our minds- 

 have died out, never to return. When, however, a type 

 is formed, it tends to specialize itself. Hence the 

 tendency that like breeds like. This tendency to pro- 

 pagate a type is called inheritance, but it is found 

 there are always slight variations, and the adapta- 

 tion to external conditions causes a slow growth of 

 alterations in form. These slow alterations are called 

 adaptations. All types have been produced by a process 

 which is now fairly understood and known as the law of 

 the struggle for existence and the survival of the fittest. 

 This is acknowledged by all naturalists, and this 



1 " It must be mentioned that very recently the important 

 theoretical question as to the nature and idea of 'kind' or ' species,' 

 which is the point 011 which really hang all the disputes about the 

 Theory of Descent, has been definitely settled. For more than a 

 century this question was discussed from the most varied points of 

 view, without resulting in a satisfactory settlement. During that 

 time thousands of zoologists and botanists have occupied themselves 

 in systematically distinguishing and describing species, without, how- 

 ever, any clear idea of the meaning of ' species.' Many hundred 

 thousand vegetable and animal forms were set up and marked as good 

 species, though even those who declared them such were unable to 

 justify the proceeding, or to give logical reasons for thus distinguishing 

 them. Endless disputes arose among the ' pure systematizers.' on the 

 empty question, whether the form called a species was ' a good or bad 

 species, a species or a variety, a sub-species or a group,' without the 

 question being even put as to what these terms really contained and 

 comprised. If they had earnestly endeavoured to gain a clear con- 

 ception of the terms, they would long ago have perceived that they 

 have no absolute meaning, but are merely stages in the classification, 

 or systematic categories, and of relative importance only." (" The 

 Evolution of Man," Prof. Ernst Haeckel, 1883, vol. i. p. 115.) 



