5 z8 



THE QUARTERLY REVIEW OF BIOLOGY 



of variants may be induced by unusual 

 conditions surrounding the animal in the 

 early stages, or they may be due to the 

 influence of such conditions on the parents. 

 Indeed, some forms of animal life are so 

 very sensitive that it is almost impossible 

 to produce the normal form under confine- 

 ment. I have found this to be the case 

 with our little carnivorous butterfly 

 (Feniseca tarquinius) . 



By far the greater number of known 

 variants are due to purely internal factors. 

 These are much more numerous than is 

 commonly supposed. For instance, in one 

 of the common feather-stars from the 

 Antarctic seas (Promachocrinus kerguelensis) 

 no less than 54 per cent of all the young 

 after they have reached the adult form 

 show features never present in the fully 

 grown. That is to say, more than half 

 of all the young produced by this particu- 

 lar animal are of such a nature that there 

 is no hope of their existence as adults 

 under the conditions they must meet. 

 These hopeless variants, however, all 

 show features occurring normally in other 

 types of crinoids found elsewhere. 



The persistence of a variant type de- 

 pends on two conditions. It must have 

 an internal balance permitting it to 

 function normally, and it must reach a 

 situation where it may exist in the face 

 of competition. If these two conditions 

 can be met, life is possible for any variant, 

 no matter how profoundly it may differ 

 from its parents, and it becomes possible 

 for it to reproduce its kind. 



In the crustaceans the most frequent ab- 

 normality is a difference in the two sides, 

 or an asymmetry, which may amount to a 

 grotesque distortion in a large percentage 

 of the young. But many of the hermit 

 crabs have succeeded in making an asset of 

 a distorted body, while other still more 

 distorted types have found existence 

 possible as parasites. 



In the crustaceans deviation from the 

 usual type has progressed so far as to give 

 rise to such anomalies as the barnacles, 

 and to a host of curious parasites of widely 

 different sorts, one of which (Thompsonid) 

 is simply a mass of structureless mycelium- 

 like roots within the tissues of the host. 



In the closely allied insects a difference 

 in the development of the two sides is 

 seldom noticed. But it is not infrequent. 

 In raising butterflies individuals are often 

 found with the wings of the two sides 

 of more or less different shape. In nature 

 these, as they cannot fly, or at least fly 

 well, are eliminated as fast as they appear. 



The ability to travel in a straight line is 

 a necessity for all animals on land, since 

 they must seek their food, or a food supply 

 for their young. It is not a necessity for 

 animals living in the sea, since if these 

 cannot seek their food the water will do 

 the work of bringing food to them. So 

 any deviation from the usual body form 

 of insects that involves a difference in the 

 development of the two sides and thus 

 hinders or prevents progress in a straight 

 line, is incompatible with the existence 

 of the individuals concerned, while in the 

 crustaceans distorted individuals may 

 somewhere find an economic niche where 

 life for them is possible. 



What is the significance of variation, 

 and what is its effect on animal life taken 

 as a whole? 



WHAT IS A SPECIES? 



Before we answer this we must first 

 define a species. The accepted definition 

 of a species is an assemblage of individuals 

 which agree with each other in form, size, 

 color and other characters, in one or more 

 of which they constantly differ from re- 

 lated assemblages of individuals, which 

 normally and freely interbreed, and which 

 transmit to their offspring their proper 

 characters unchanged, or with that little 



