GENERAL LAW OF ANIMAL FERTILITY. 591 



Higher organisms are distinguished from lower ones partly by- 

 bulk, and partly by complexity. This complexity essentially 

 consists in the mutual dependence of numerous different organs, 

 each subserving the lives of the rest, and each living by the help 

 of the rest. Instead of being made up of many like parts, per- 

 forming like functions, as the Crinoid, the Star-fish, or the Milli- 

 pede, a vertebrate animal is made up of manv unlike parts, 

 performing unlike functions. From that initial form of a com- 

 pound organism, in which a number of minor individuals are 

 simply grouped together, we may, more or less distinctly, trace 

 not only the increasing closeness of their union, and the gradual 

 disappearance of their individualities in that of the mass, but 

 the gradual assumption by them of special duties. And this 

 " physiological division of labour," as it has been termed, has 

 the same effect as the division of labour amongst men. As the 

 preservation of a number of persons is better secured when, 

 uniting into a society, they severally undertake different kinds 

 of work, than when they are separate and each performs for him- 

 self every kind of work ; so the preservation of a congeries of 

 parts, which, combining into one organism, respectively assume 

 nutrition, respiration, circulation, locomotion, as separate func- 

 tions, is better secured than when those parts are independent, 

 and each fulfils for itself all these functions. 



But the condition under which this increased ability to main- 

 tain life becomes possible is, that the parts shall cease to separate. 

 While they are perpetually separating, it is clear that they cannot 

 assume mutually subservient duties. And it is further clear that 

 the more the tendency to separate diminishes, that is, the larger 

 the groups that remain connected, the more minutely and perfectly 

 can that subdivision of functions which we call organization he car- 

 ried out. 



Thus we see that in its most active form the ability to multiply 

 is antagonistic to the ability to maintain individual life, not only 

 as preventing increase of bulk, but also as preventing organiza- 

 tion — not only as preventing homogeneous co-ordination, but as 

 preventing heterogeneous co-ordination. 



§ 10. To establish the unbroken continuity of this law of 

 fertility, it will be needful, before tracing its results amongst the 

 higher animals, to explain in what manner spontaneous fission is 

 now understood, and what the cessation of it essentially means. 

 Originallv, naturalists supposed that creatures which multiply by 

 self-division, under any of its several forms, continue so to 

 multiply perpetually. In many cases, however, it has latterly 

 been shown that they do not do this ; and it is now becoming a 

 received opinion that they do not, and cannot, do this, in any 



