418 EVOLUTION AND GENETICS 



The student should note that questions of this kind may be 

 very intricate and may permit apparently sound conclusions which 

 are, in reahty, of no value whatsoever. No better example of the 

 difficulty is available than a recent treatment of this same question. 

 The skin on the soles of the feet is thickened in individuals in 

 proportion to the friction between the feet and the substratum. 

 Walter points out that even in the mud-puppies {Necturus, a 

 genus of tailed amphil)ia) the skin of the soles is thickened, 

 although these animals always live in the water and therefore 

 cannot exert much pressure on the soles of the feet. He adds: 

 "Nor is it reasonable to suppose that it ever had any ancestor who 

 did so for the hands and feet of the Amphil)ia are the most primitive 

 and ancient hands and feet to be found in the animal kingdom 

 without any known ancestral types. The thickening of the skin 

 on the sole of the mud-puppies' feet must be due, therefore, to germi- 

 nal determiners and is in no way an acquisition through use." 

 Detlefsen quotes this passage, inserting the italics which are here 

 reproduced, and asks: "But is it really proven that the ancestors 

 never habitually applied pressure or friction to the soles of their 

 feet? Or do we know that the small amount of friction which 

 Necturus develops as it crawls along the bottom or behind rocks 

 and stones, would not develop some skin thickening?" These 

 two opinions on one point are an excellent illustration of individual 

 fallibility, for as we examine the evidences of evolution we find 

 that the pentadactyl appendage first appears as an adaptation to 

 terrestrial life. If this generally accepted point is true, then no 

 animal with pentadactyl appendages or their derivatives can fail 

 to have had terrestrial or amphil:)ious ancestors. At some time 

 in the past, therefore, Necturus must have had ancestors in which 

 the feet supported the body on solid earth during at least a part 

 of their lives, and here is the desired source of friction! It is 

 therefore just as inaccurate to say that the thickened soles of 

 Necturus cannot be due to the individual acquisitions of long ago 

 as to say definitely that they were so produced. Such evidence is 

 simply inconclusive. 



Evidences of this kind may be explained just as readily by the 

 Darwinian theory. It is quite as accurate to say that a certain 

 useful character appears fortuitously in some individuals and is 

 perpetuated by natural selection as to say that it developed in 

 the individual as an acquired character and because of its value 



