292 STUDIES, SCIENTIFIC AND SOCIAL chap. 



abundant food may necessitate some modification in 

 structure better adapted to secure it, or the means of 

 ranging over a wider area to search for it ; while a 

 severer climate may necessitate a thicker covering, or more 

 nourishing food, or new kinds of shelter. To bring about 

 these changes, " variation " and the " struggle for existence" 

 come into play. Each year the forms less adapted to the 

 altered conditions die out, while those variations which 

 are more in harmony with them constantly survive ; and 

 this process, continued for many hundreds or thousands of 

 successive generations, at length results in the production 

 of one or more new species. 



Variahility. 



We now come to the difficulty which has been repeatedly 

 put forward, and which seems very great to all who have 

 not studied groups of species as they occur in nature, 

 and which is expressed in the question " How comes it 

 that variations of the right kind and sufficient in amount 

 have always occurred just when they were wanted, so as 

 to form the endless series of new species that have 

 arisen ? " and it is more especially to answer this question 

 that the present chapter has been written. 



Few persons consider how largely and universally all 

 animals are varying; yet it is certain that if we could 

 examine all the individuals of any common species we 

 should find considerable differences, not only in size and 

 colour but in the form and proportions of all the parts and 

 organs of the body. In our domesticated animals we 

 know this to be the case, and it is by means of the con- 

 tinued selection of such slight varieties to breed from 

 that all our extremely varied domestic animals and cul- 

 tivated plants have been produced. Think of the differ- 

 ence in every limb, in every bone and muscle, and 

 probably in every part, internal and external, of the whole 

 body, between a greyhound and a bull-dog ! Yet, if we 

 had the whole series of ancestors of these two breeds 

 before us, we should find them gradually converge till 

 they reached the same original type, while between no two 

 successive generations would there be any greater difference 



