MODERN BIOLOGY 467 



is not essentially different from the term "variety," which is used for less 

 distinct and more iluctuating forms. 



Variations in progeny 

 The causes of these variations, which by means of selection — natural in 

 wild life, human in domestic animals — are developed into varieties and 

 species, involve a problem that occupied the mind of Darwin a great deal. 

 He at once points out that only hereditary variations have any significance 

 and also that the essential causes of them have never been really ascertained. 

 On this question he adopts a somewhat hesitant attitude; it is true, he as- 

 serts, and collects ample material to prove, that external conditions pro- 

 duce variations in the progeny, which is a view strongly reminiscent of 

 Lamarck, but, on the other hand, he definitely rejects all Lamarckian ideas. 

 As a matter of fact, this theory of the heredity of variations is the basis 

 of the Darwinian theory, but it is also one of its weakest points; in this 

 connexion modern research into the problem of heredity has passed severe 

 judgment on him — often indeed unfairly severe, it being forgotten that he 

 had not that accumulation of facts to build upon which is available in modern 

 times, but here he certainly does touch upon extremely vague conceptions, 

 which make the chapter on the law of variation difficult to comprehend. 

 Among the circumstances that influence the individual's reproductive or- 

 gans and thus affect the offspring, he mentions climatic conditions of vari- 

 ous kinds and alimental conditions, as well as the correlation between 

 different parts of the body. Nevertheless, he always insists upon the im- 

 portance of natural selection as being greater than the direct influence of 

 environment. For instance, a number of insect forms on islands in mid-ocean 

 have restricted powers of flight as compared with their relations on the 

 mainland; this has arisen through the fact that those specimens that are 

 best at flying have been blown out to sea and perished, while the wxaker 

 fliers have continued to propagate, rather than through the animals' not 

 having dared to use their wings, with the result that their growth has be- 

 come stunted. Correlation, again, compels other organs to follow suit when 

 one organ has been modified as a result of selection. Further, he holds that 

 parts of the body that have become especially developed in one species, as 

 compared with corresponding parts in closely related species, are liable to 

 peculiar variation; thus, the length of the arms of the orang-utan varies, 

 just as, in general, every strongly developed characteristic indicates strong 

 variation in the previous generation. On the other hand, the wings of the 

 bat do not vary, abnormal though they are in comparison with the extremi- 

 ties of other mammals, for the entire group has wings of the same kind; 

 the law would hold good only if one species had longer pairs of wings than 

 other species of the same genus. On these grounds he believes also that 

 species-characters vary more than genus-characters, but the variations of 



