THE LAW OF EVOLUTION. 325 



even in the lower quadrumana the uterus is somewhat cleft 

 at its summit." * 



Under the head of organic integrations, there remain to 

 be noted some which do not occur within the limits of one 

 organism, and which only in an indirect way involve con- 

 centration of matter and dissipation of motion. These are 

 the integration by which organisms are made dependent on 

 one another. We may set down two kinds of them those 

 which occur within the same species, and those which occur 

 among different species. More or less of the gre- 



garious tendency is general in animals; and when it is 

 marked, there is, in addition to simple aggregation, a certain 

 degree of combination. Creatures that hunt in packs, or 

 that have sentinels, or that are governed by leaders, form 

 bodies partially united by co-operation. Among polyga- 

 mous mammals and birds this mutual dependence is closer; 

 and the social insects show us assemblages of individuals of 

 a still more consolidated character: some of them having 

 carried the consolidation so far that the individuals cannot 

 exist if separated. How organisms in general are 



mutually dependent, and in that sense integrated, we shall 

 see on remembering first, that while all animals live 

 directly or indirectly on plants, plants live on the carbonic 

 acid excreted by animals; second, that among animals the 

 flesh-eaters cannot exist without the plant-eaters; third, 

 that a large proportion of plants can continue their respec- 

 tive races only by the help of insects, and that in many 

 cases particular plants need particular insects. Without 

 detailing the more complex connexions, which Mr. Darwin 

 has so beautifully illustrated, it will suffice to say that the 

 Flora and Fauna in each habitat, constitute an aggregate 

 so far integrated that many of its species die out if placed 

 amid the plants and animals of another habitat. And it 

 is to be remarked that this integration, too, increases as 

 organic evolution progresses. 



* Carpenter's Prin. of Comp. Phys., p. 617. 



