182 THE NATURALIST IN NICARAGUA. [Ch. X. 



ingly abundant, and very destructive to the fruit. In 

 the next spring, numerous females that had hyhernated 

 commenced making their paper nests, and I anticipated 

 a still greater plague of wasps in the autumn than we 

 had had the year before ; but some epidemic carried off 

 nearly all the females before they finished building their 

 nests, and in the autumn scarcely a wasp was to be seen ; 

 I saw also in the Natural History magazines notices of 

 their scarcity in all parts of England. 



The great mortality amongst the insects of Chontales 

 in 1872 has some bearing on the origin of species, for in 

 times of such great epidemics we may suspect that the 

 gradations that connect extreme forms of the same 

 species may become extinct. Darwin has shown how 

 very slight differences in the colour of the skin and hair 

 are sometimes correlated with great immunity from 

 certain diseases, and from the action of some vegetable 

 poisons, and the attacks of certain parasites.* Any 

 varieties of species of insects that could withstand better 

 than others these great and probably periodical epi- 

 demics, would certainly obtain a great advantage over 

 those not so protected ; and thus the survival of one 

 form, and the extinction of another, might be brought 

 about. We see two species of the same genus, as in 

 many insects, differing but little from each other, yet 

 quite distinct, and we ask, why if these have descended 

 from one parent form, do not the innumerable grada- 

 tions that must have connected them exist also ? There 

 is but one answer ; we are ignorant what characters are 



* " Descent of Man," vol. i. p. 242 ; and " Animals and Plants 

 under Cultivation," vol. ii. pp. 227-230. I have taken the examples 

 given from the same author. 



