82 THE PEINCIPLES OF HEREDITY 



fact that species removed to new environments tend to become 

 more variable in every way. Consider a plant growing wild 

 in a wood. It fills a particular niche in nature, which if it 

 alter much it will not fit. Working together for ages, Natural 

 Selection and sexual reproduction, progressive and regressive 

 evolution, have closely adapted it in all its parts to the 

 environment. Natural Selection has met every gradual in- 

 crease of stringency in the conditions under which the species 

 lives by slowly evolving new structures, or much more often 

 by modifying old structures. Sexual reproduction has met 

 every decrease of stringency by planing away all redundancies. 

 When the adaptation to the little-changing environment has 

 nearly approached perfection and excessive variation has be- 

 come disadvantageous, the two forces have united to preserve 

 the stability of the type ; Natural Selection by eliminating 

 regressive variations, and sexual reproduction by eliminating 

 excessive progressive variation. Since the environment in 

 wild nature changes very slowly, an appearance of great 

 stability and uniformity in all the individuals of the species 

 is thus prod uced. And not only an appearance. It is plain, 

 whenever the adaptation to the environment is close, that 

 Natural Selection and sexual reproduction will combine to 

 limit the tendency to vary; Natural Selection by destroying 

 individuals that tend to vary greatly, sexual reproduction by 

 reducing this tendency in any offspring such individuals may 

 chance to have towards the specific mean. It must be 

 remembered that an unusual tendency to vary is itself a varia- 

 tion, for, judging by analogy, it is not at all probable that all 

 individuals tend to vary to exactly the same extent ; and if 

 they do not vary to the same extent, Natural Selection must 

 work on these variations just as it does on any other, and so 

 exalt or reduce the tendency according to the needs of the 

 time. Every species fills a particular niche in nature. But 

 that niche changes its shape, at times with comparative quick- 

 ness, at times very slowly. The species follows suit. When the 

 niche is changing rapidly great variations are advantageous, 

 and Natural Selection therefore chooses for survival those 

 individuals who vary greatly (and favourably). When the 

 niche is nearly stable great variations are disadvantageous. 

 They do not, then, increase the adaptation ; they merely in- 

 crease the elimination. At such times, therefore, Natural 

 Selection chooses for survival those individuals that tend to 

 vary but little. These restrictions on the tendency to vary 

 are probably of great importance, but have not received the 

 attention they deserve. 



