192 CHAPTERS ON EVOLUTION. 



scientific explanation by the further assertion that, insects being 

 voracious in their feeding habits, especially in earlier life, perform an 

 important function in the economy of nature in that they remove 

 from the earth's surface " superabundant and decaying animal and 

 vegetable matter." A further reason for this providential arrange- 

 ment was given in the fact that, as " unusual powers of multipli- 

 cation " were indispensable for recruiting the ranks of the insect 

 scavengers, and as nutrition and reproduction are incompatible 

 functions, the removal of decaying matter during the youthful stages 

 of the insect's life was to be regarded as a convenient subdivision 

 of its labours, seeing that its adult existence is spent in the work of 

 reproducing its race. But it might easily be shown that, whilst a 

 goodly number of larval insects do feed upon carrion, a large pro- 

 portion of the class does not exhibit any such habit ; and it might 

 reasonably enough be maintained that the argument of Kirby and 

 Spence is open to the serious objection that, in its character, it 

 tends to illustrate the post hoc ergo propter hoc fallacy Decaying 

 matter exists, therefore insects were designed to pass through a 

 metamorphosis, and were gifted with voracity of disposition that 

 they might remove the said matter from the earth's surface a 

 proposition vitiated in its exactitude by the fact just mentioned that 

 many insects do not eat such matter; and also by the further facts 

 that many do not undergo a metamorphosis at all ; that many vora- 

 cious caterpillars, instead of eating decaying matter, destroy our trees 

 and flowers. It might also be added that many of nature's scavengers 

 of higher and lower rank than the insects, do not pass through a 

 series of changes in development, but grow, nourish themselves in 

 the exercise of their sanitary work, and likewise, at the same time, and 

 as adult forms, reproduce their species and continue their race in time. 

 Clearly, then, the explanation of Kirby and Spence affords no satis- 

 faction to the contemplative mind in the natural anxiety and desire 

 to discover the causes of things. At its very best, such explanation 

 leaves " the reason why " untouched ; and conversely, it can well be 

 understood how any other system of thought, which presents a more 

 satisfactory method of accounting for the facts in question, should find 

 ready acceptance as expanding and enlarging the thoughts of men. 



In the previous chapter we discussed the meaning of the remark- 

 able likenesses which can be readily proved, as matters of fact and 

 observation, to exist between the early stages in the development of 

 very different animals. A sponge, a sea-squirt, a lancelet, and even 

 higher animals still, appear in the first beginnings of their existence to 

 pursue a remarkably similar course. Each form parts company with 

 its fellows at a given stage on the way of development, and thereafter 

 passes by the special pathway of its race towards the adult and perfect 

 stage. Von BaeVs axiom that development proceeds from the general 



