NATURAL SELECTION 153 



struggle for existence, yet certain characters which, though 

 useless are harmless, sometimes survive and remain constant, or 

 fairly constant. Hence the extraordinary number of species 

 (some of them probably mere varieties) of willows and briers in 

 the British Isles and of shells in the Sandwich Islands. 



The variations on which Natural Selection has to work are 

 usually small, but occasionally large. Even very slight differ- 

 ences may cause survival or destruction. 



There is not wanting evidence that the tendency to retro- 

 gression is, but for the constant elimination of the inferior, 

 stronger than the forward tendency. Weismann, therefore, is 

 right in holding that Pammixis can undo the work of Natural 

 Selection even without the aid of reversed selection. 



The shedding of characters that have become useless and 

 cumbersome is one of the conditions of evolution. The addition 

 of new organs is accompanied by the disappearance or reduction 

 to the minutest dimensions of such as have become obsolete. 



A number of species are evolved simultaneously. A forward 

 step in one necessitates an advance in those that come in contact 

 with it. 



Nothing but change of environment can lead to further evolu- 

 tion. When the conditions remain the same, elimination tends 

 only to produce organic stability. 



The pace of evolution has varied at different periods. It is 

 possible that among certain species at certain times in certain 

 regions a condition of equilibrium may be attained so that in 

 these species nothing beyond greater stability may be in process 

 of evolution. On all sides now there are highly specialised 

 organisms and this makes it impossible that the circumstances 

 under which evolution began can ever be reproduced. In the 

 lower species much of the elimination is indiscriminate, the fit 

 and the unfit being alike destroyed. The amount of such waste 

 is much reduced among the higher plants by machinery which 

 secures the safe transference of pollen and the scattering of their 

 seeds. Among the higher animals waste is checked by a far 

 more effective system, by the care of parents for their offspring 

 during their immaturity. The struggle comes when growth has 



