250 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



butterfly, Kallima, previously described (pp. 201, 202). In its present 

 condition this animal has a strikingly detailed resemblance to a dried 

 leaf, which is therefore doubtless of some value. But of what value 

 would be the first tiny change in the direction of resemblance ? Until 

 its resemblance became close enough actually to deceive the enemies of 

 butterflies, the critics claim, there would be no chance for selection to act. 



8. It is frequently objected that a vast number of characters of 

 organs are useless or non-adaptive and, as such, could not have arisen 

 through the instrumentality of natural selection. If these useless 

 characters, which are sometimes quite large and prominent, are 

 independent of natural selection, why do we need natural selection to 

 explain adaptive characters? It is also claimed that a vast number 

 of specific peculiarities are useless and therefore could not have helped 

 in the differentiation of species. It should be said in defense that 

 Darwin realized this difficulty quite as clearly as do his critics and 

 was greatly puzzled by it. His idea of correlated variability, however, 

 helps to answer it, for it may well be that many of these apparently 

 useless characters are correlated, or Hnked in inheritance, with charac- 

 ters -of supreme selective value such as general hardiness or great 

 fecundity. Darwin also points out that we are not in a position at 

 present to pronounce judgment on the value of many structures or 

 functions that have been adjudged non-adaptive. 



g. Certain characters in organisms, past and present, have been 

 interpreted as overspecializations, organs that have evolved beyond 

 the range of usefulness or that are more elaborate than is demanded 

 for survival under the conditions of life. The case of the extinct Irish 

 elk is often cited as an example of overspecialization. This group of 

 animals went to extremes in the development of size and elaboration 

 of horns far beyond the range of usefulness, so that it is said to have 

 brought about the extinction of the race. Natural selection, which is 

 supposed to have brought the horns up to the point of adaptive 

 perfection, should have kept them within the bounds of usefulness. 



Again, the enormously overgrown and overspecialized dinosaurs 

 of long ago are thought of as having followed their lines of evolution 

 far beyond the point of greatest effectiveness and adaptability. 



10. The rudimentation of structures, which is such a common 

 phenomenon in nature, is said to meet with no adequate explanation 

 on a selection basis. The case of the whale's vestigial hind limbs is 

 a case in point. Darwin's explanation would be that under aquatic 

 conditions the first whale ancestors would be handicapped by hind 



