280 



GENERALIZATIONS. 



when its time has elapsed ; but so long as we are unable to 

 indicate a cause in the nature of the species for the limitation 

 of time, there must be uncertainty in forming such limits of 

 duration. 



Darwin, in his celebrated book ' On the Origin of Species by 

 Means of Natural Selection/ has endeavoured, in a most inge- 

 nious manner, to explain and combine the extinction of species 

 and the origin of new ones. He starts from the ascertained fact 

 that the number of individuals of plants and animals increases 

 in geometrical ratio, which is not the case with their nutritive 

 materials. A great number of plants and animals are therefore 

 deprived of the space and nutriment which they require for life, 

 and many must annually be destroyed. Thus a constant struggle 

 for existence takes place, and is even produced between indivi- 

 duals of the same species, of which only a few attain maturity. 

 If one of these individuals possesses any advantage over the 

 others, it will more easily survive, whilst the weaker ones will 

 languish and die. The former can bequeath its advantages to 

 its posterity, and, when they increase in a definite direction, in- 

 dividuals may gradually be produced differing considerably from 

 the first, and forming a new race. 



This new race Darwin regards as a young species in course of 

 formation ; for if the development continues in the same direc- 

 tion, the difference by the continual accumulation during thou- 

 sands of generations of deviations so small as to be scarcely per- 

 ceptible, may become so great as to form what we call a species. 

 The old species, however, would have disappeared because it 

 could not sustain the competition with its younger and stronger 

 descendants. Consequently old species would have been extin- 

 guished, and new ones produced in all cases where some of the 

 younger individuals had acquired properties specially favourable 

 to their continued development, and had transmitted these pro- 

 perties to their progeny, and, in consequence, displaced their 

 unchanged relations. But the individuals of a species may also 

 in course of time become developed in various directions, so 

 that a whole group of species may originate from the divergent 

 forms, and this is regarded as a genus. All the species of a 

 genus would therefore have a species as their starting-point. 

 But if we go still further back, the genera also coalesce ; and 



