190 DARWINISM TO-DAY. 



the most ingenious mechanical explanation yet offered of 

 the workings of such a factor. Indeed, Weismann, with 

 characteristic ingenuity and capacity, has offered the be- 

 lievers in orthogenesis that which they so far had not been 

 able to get for themselves, namely, a possible causo-mechan- 

 ical explanation of it. It should be noted that Roux's theory 

 of the battle of the parts (explained later) was a forerunner 

 of, and undoubtedly the suggestion for, the theory of 

 germinal selection. 



Familiar to all students of biology, and certainly not 



wholly unfamiliar to laymen, are those structures or parts 



in the body known variously as vestigial struc- 



Ehe theory of tures rudimentary or degenerate organs. The 



panmixia to ex- J 



plain vestigial vermiform appendix in man is one; the eye of 



the mole is another ; the functionless wing of 

 the ostrich, the useless fore-feet of a milk-weed butterfly, 

 and the splint bones of the horse, are others. Almost every 

 animal kind possesses vestigial organs, and some kinds 

 possess very many. Those in the human body make an 

 amazingly long list. All these are organs, which have once 

 that is, in ancestors of the present particular organism 

 been well-developed and probably useful. But these 

 organs now are useless or even harmful. The human 

 appendix vermiformis is harmful ; the tiny fore-feet of the 

 milk-weed butterfly are useless. Why do animals have 

 such vestigial organs? Because they derive them by hered- 

 ity from ancestors. But in these ancestors the organs were 

 well developed and useful. How is it that the present 

 organisms do not need the same organs ? They have adopted 

 new habits, or live in a new environment, or have developed 

 other means of supplying the old want ; in a word the organs 

 are superfluous. How is it that the organs have become 

 thus degenerate or vestigial? This is the question that 

 selection has difficulty in answering satisfactorily. Selec- 

 tion can develop and specialise organs of use and advantage ; 



