272 



THE BRITISH NATURALIST. 



[November 



THE DEVELOPMENT OF SOCIAL INSTINCTS. 



BY A. J. JOLLEY. 



( Continued from page 247.) 



Passing to the Placental mammals, which are classified into thirteen 

 orders, we find in the lowest of these, the Edentata, which includes 

 the ant-eaters, sloths and armadilloes, as well as in the much higher 

 order, Insectivora, i.e., hedgehogs, moles, shrews, and the like, no 

 social species ; the Edentates are too stupid, and the Insectivores too 

 ferocious, for even the most rudimentary form of sociality to exist 

 amongst them. 



Amongst the numerous and widely-distributed species which con- 

 stitute the order Rodentia, there is considerable diversity of structure, 

 of habit, and of intelligence, and the development of the latter is in 

 some species manifestly correlated with increased sociality. A high 

 rate of reproduction, with a plentiful food supply and a generally 

 suitable environment, leads in some cases to the aggregation of large 

 numbers on small areas. Of this, the common rabbit supplies an 

 illustration ; though, except so far as the presence of considerable num- 

 bers renders the unobserved approach of enemies more difficult, since 

 one startled animal warns the others of the prospective danger, rabbits 

 can scarcely be called social animals. Much the same holds of the 

 South American vizcachas, whose numerous burrows render riding on 

 the undermined surface of the Pampas a work of no little difficulty and 

 danger. These South American rodents are, however, both more 

 intelligent and more social than our rabbits ; they not only play 

 together and warn one another of impending peril, but sometimes unite 

 to resist their enemies. There is at least rudimentary sociality amongst 

 many species of cavies, chinchillas, marmots, jerboas, voles, rats and 

 mice, but the greatest development of the social instincts and the 

 highest level of intelligence amongst the Rodents is found in the 

 beavers, animals which date from the Miocene age, and which have 

 had a very wide range in the northern hemisphere. Some beavers live 

 in burrows with one end opening under water, and this may fairly be 

 deemed the primitive type of beaver dwelling, from which the others 

 have been developed. The storing of timber near the burrow (for 

 the beaver is mainly a bark feeder) probably originated the idea of 

 a dwelling-place into which the burrow might open on the land side ; 

 from this to the oven-shaped lodge, built of strong timbers with inter- 

 woven twigs and grass and a thick coating of mud," the transition is not 

 very difficult. The erection of such lodges on island, river-bank, or 

 lake-shore, or rising out of some quiet shallow, evidences the 

 intelligence of the builders, who adapt themselves to circumstances, 



