G20 APPENDIX B. 



habits. The change from the structaros of hind limbs fitted only 

 for walking and trotting to hind limbs fitted also for leaping, 

 implies, therefore, that, along with strengtheniags of bones there 

 must go alterations in their forms. Now the fortuitous altera- 

 tions of form which may take place in any bone are countless. 

 How long, then, will it be before there takes place that particu- 

 lar alteration which will make the bone fitter for its new action ? 

 And what is the probability that the many required changes of 

 shape, as well as of size, in bones will each of them be effected 

 before all the others are lost again ? If the probabilities against 

 success are incalculable, when we take account only of changes in 

 the sizes of parts, what shall we say of their incalculableness when 

 differences of form also are taken into account ? 



" Surely this piling up of difficulties has gone far enough " ; 

 the reader will be inclined to say. By no means. There is a 

 difficulty immeasurably transcending those named. We have thus 

 far omitted the second half of the leap, and the provisions to be 

 made for it. After ascent of the animal's body comes descent ; 

 and the greater the force with which it is projected up, the 

 greater is the force with which it comes down. Hence, if the 

 supposed creature has undergone such changes in the hind limbs 

 as will enable them to propel it to a greater height, without 

 having undergone any changes in the fore limbs, the result will 

 be that on its descent the fore limbs will give way, and it will 

 come down on its nose. The fore limbs, then, have to be 

 changed simultaneously with the hind. How changed ? Con- 

 trast the markedly bent hind limbs of a cat with its almost 

 straight fore limbs, or contrast the silence of the spring on to the 

 table with the thud which the fore paws make as it jumps off the 

 table. See how unlike the actions of the hind and fore limbs 

 are, and how unlike their structures. In what way, then, is the 

 required co-adaptation to be efl'ected ? Even were it a question of 

 relative sizes only, there would be no answer ; for facts already 

 given show that we may not assume simultaneous increases of 

 size to take place in the hind and fore limbs; and, indeed, a 

 glance at the various human races, which differ considerably in 

 the ratios of their legs to their arms, shows us this. But it is 

 not simply a question of sizes. To bear the increased shock of 

 descent the fore limbs must be changed throughout in their 

 structures. Like those in the hind limbs, the changes must be 

 of many parts in many proportions ; and they must be both in 

 sizes and in shapes. More than this. The scapular arch and its 

 attached muscles must also be strengthened and re-moulded. See, 

 then, the total requirements. We must suppose that by natural 

 selection of miscellaneous variations, the parts of the hind limbs 

 will be co-adapted to one another, in sizes, shapes, and ratios ; 



