(512 APPENDIX B. 



massive tail of the kangaroo to become useless (say by the forcing 

 of the species into a mountainous and rocky habitat filled with 

 brushwood), a variation which considerably reduced the tail 

 might sensibly profit the individual in which it occurred ; and, in 

 seasons when food was scarce, might cause survival when indi- 

 viduals with large tails died. But the economy of nutrition must 

 be considerable before any such result could occur. Suppose 

 that in this new habitat the kangaroo had no enemies ; and sup- 

 pose that, consequently, quickness of hearing not being called 

 for, large ears gave no greater advantage than small ones. Would 

 an individual with smaller ears than usual, survive and propagate 

 better than other individuals, in consequence of the economy of 

 nutrition achieved ? To suppose this is to suppose that the sav- 

 ing of a grain or two of protein per day would determine the 

 kangaroo's fate. 



Long ago I discussed this matter in the Principles of Biology 

 ( 166), taking as an instance the decrease of the jaw implied by 

 the crowding of the teeth, and now proved by measurement to 

 have taken place. Here is the passage : 



" No functional superiority possessed by a small jaw over a large jaw, in 

 civilized life, can be named as having caused the more frequent survival of 

 small-jawed individuals. The only advantage which smallness of jaw might 

 be supposed to give, is the advantage of economized nutrition ; and this 

 could not be great enough to further the preservation of men possessing it. 

 The decrease of weight in the jaw and co operative parts that has arisen in 

 the course of many thousands of years, does not amount to more than a few 

 ounces. This decrease has to be divided among the many generations that 

 have lived and died in the interval. Let us admit that the weight of these 

 parts diminished to the extent of an ounce in a single generation (which is a 

 large admission) ; it still cannot be contended that the having to carry an 

 ounce less in weight, or having to keep in repair an ounce less of tissue, could 

 sensibly affect any man's fate. And if it never did this nay, if it did not 

 cause a frequent survival of small-jawed individuals where large-jawed indi- 

 viduals died, natural selection could neither cause nor aid diminution of the 

 jaw and its appendages." 



When writing this passage in 1864, I never dreamt that a 

 quarter of a century later, the supposable cause of degeneration 

 here examined and excluded as impossible, would be enunciated 

 as an actual cause and named " reversed selection." 



One of the arguments used to show the adequacy of natural 

 selection under its direct or indirect form consists of a counter- 

 argument to the effect that inheritance of functionally-wrought 

 changes, supposing it to be operative, does not explain certain of 

 the facts. This is alleged by Prof. Weismann as a part justification 

 for his doctrine of Panmixia. Concerning the " blind fish and 

 amphibia " found in dark places, which have but rudimentary eyes 

 " 1'idden under the skin," he argues that " it is difficult to reconcile 

 the facts of the case with the ordinary theory that the eyes of these 



