DARWINISM AND EVOLUTION. 1 3 



But if the environment changed more suddenly, varied indi- 

 viduals would be found ready to take advantage of it. 



"Consider the glacial epoch of 260,000 years, beginning about 

 " 980,000 years ago. During that long period," says Mr. Croll, 

 " the changes from cold to warm conditions of climate every 

 "10,000 or 12,000 years must have been of the most extreme 

 "kind. Compare that with the period beginning, say, 80,000 

 "years ago, and extending to nearly 150,000 years into the future, 

 " during which there will be no extreme variations of climate, and 

 " how great is the contrast ! How extensive the changes in 

 " species must have been during the first period, as compared 

 " with those that are likely to take place during the latter." 



In this argument Mr. Croll confines his view to the influence 

 of climate, which is, indeed, but a part of an animal's environment. 

 The most important portion of the present circumstances of man- 

 kind is undoubtedly the keenness of the mutual struggle for 

 existence. 



8. The eighth proposition is that changes due to individual 

 variation are complicated by a law that a change set up in any 

 one part of an organism initiates changes in other parts. This 

 phenomenon is generally spoken of as " correlated variation." 



Proof is afforded even by the conditions of ordinary growth. It 

 is a fact in physics that in similarly shaped bodies the masses vary 

 as the cubes of the dimensions, whereas the strengths vary as the 

 squares of the dimensions. 



" Supposing," remarks Mr. Spencer, " that a creature which a 

 " year ago was one foot high, has now become two feet, while it 

 " is unchanged in proportions and structure, what are the necessary 

 " concomitant changes that have taken place in it ? It is eight 

 " times as heavy ; that is to say, it has to resist eight times the 

 " strain which gravitation puts on its structure ; and in producing, 

 " as well as arresting, every one of its movements, it has to over- 

 " come eight times the inertia. Meanwhile, the muscles and bones 

 " have severally increased their contractile and resisting powers in 

 " proportion to the areas of their transverse sections, and hence 

 " are severally but four times as strong as they were. So, while 

 " the creature has doubled in height, and while its ability to 

 " overcome force has quadrupled, the forces it has to overcome 

 " have grown eight times as great." 



Further, in order that an animal may have some structure 

 specially and largely developed, it is indispensable that several 

 other parts be modified and co-adapted. 



That the elk's enormous antlers may grow, they require an 

 increased blood supply. The arteries of the skull must therefore 

 enlarge, and the balance of the circulation must be readjusted. 

 As the head becomes heavier, by 60 pounds, with the added 

 weight of the horns, more powerful muscles and ligaments are 



