8o8 THE POPULAR SCIENCE MONTHLY. 



tion will increase any advantageous trait. Certainly a view now 

 widely accepted assumes as much. 



The consideration of this view, to which the foregoing para- 

 graph is introductory, may now be entered upon. This view con- 

 cerns, not direct selection, hut what has been called, in question- 

 able logic, " reversed selection "■ — the selection which effects, not 

 increase of an organ, but decrease of it. For as, under some con- 

 ditions, it is of advantage to an individual and its descendants to 

 have some structure of larger size, it may be, under other condi- 

 tions — namely, when the organ becomes useless — of advantage to 

 have it of smaller size; since, even if it is not in the way, its 

 weight and the cost of its nutrition are injurious taxes on the 

 organism. But now comes the truth to be emphasized. Just as 

 direct selection can increase an organ only in certain cases, so can 

 reversed selection decrease it only in certain eases. Like the in- 

 crease produced by a variation, the decrease produced by one 

 must be such as will sensibly conduce to preservation and multi- 

 plication. It is, for instance, conceivable that were the long and 

 massive tail of the kangaroo to become useless {say by the forc- 

 ing 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 suppose 

 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 kan- 

 garoo's fate. 



Long ago I discussed this matter in the Principles of Biology 

 (§ 1G6), 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 : — 



k ' No functional superiority possessed by a small jaw over a large jaw, in civ- 

 ilized life, can be named as baving caused the more frequent survival of small- 

 jawed individuals. Tbe only advantage which smallness of jaw might be sup- 

 posed 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 



