NEW TYPES AMONG ANIMALS 163 



to the standard type of their class. In unusually warm or cold 

 water, on the contrary, aberrant types are numerous. This looks 

 as if the warm or cold conditions caused variations, while the 

 optimum allows the standard type to reproduce itself unchanged. 

 That this is really the case is further suggested by various other 

 cases cited by Clark. For example, the asymmetrical narwhal is 

 exclusively arctic, while another group of equally aberrant whales 

 lives within the tropics. The anthropoid apes, which are pro- 

 nouncedly left- or right-handed, live in very warm regions ; the 

 hornbill Rhino plax, with an uneven tail, a solid casque, a naked 

 patch on the back of the head, and other peculiarities, is found in 

 warm Malayan regions, while the crossbills with the tips of the 

 mandibles crossed and a corresponding distortion of the bones of 

 the head are all subarctic or cold temperate forms. So, too, 

 Clark points out that the owls with one ear greatly larger than 

 the other all appear to inhabit cold regions. Among the fishes the 

 very asymmetrical Anableps lives in the warm tropical littoral, 

 and the asymmetrical forms of "Amphioxus" (using the term in 

 its broadest sense) occur in warm regions ; while the flatfishes are 

 chiefly developed either in warm tropical waters or in those that 

 are cold. 



Since the great majority of animals are symmetrical, any de- 

 parture from symmetry attracts attention and makes this quality 

 an easy one to study. Lack of symmetry may or may not be im- 

 portant in the life history of the animal, but it is at least an indi- 

 cation that the creature has suffered some mutation. The chances 

 are large that other mutations also accompany the changes 

 in symmetry. Among the crinoids Clark's work seems to demon- 

 strate that this is the case. Hence the relative abundance of 

 asymmetrical or aberrant forms in temperatures which depart 

 far from the optimum seems to suggest that mutations are actually 

 due to extremes of temperature. If this is so, it may have a 

 most important bearing upon the powers and capacities of man- 

 kind. 



