GENERAL LAW OF ANIMAL FERTILITY. 587 



up of cells " each of which has the character of a distinct ani- 

 malcule, having a certain power of spontaneous motion, obtaining 

 and assimilating its own food, and altogether living by and for 

 itself ; " and so small is the cohesion of these individual cells, 

 that the tissue they constitute " drains away when the mass is 

 removed from the water, like white of egg." * 



Of course in proportion as the aggregate tendency leading to 

 the formation of these groups of monads is strong, we may 

 expect that, other things equal, the groups will be large. 

 Proceeding upwards from the yeast fungus, whose cells hold 

 together in groups of four, five, and six,f there must be found 

 in each species of these composite organisms a size of group 

 determined by the strength of the aggregative tendency in that 

 species. Hence we may expect that, when this limit is passed, 

 the group no longer remains united, but divides. Such we find 

 to be the fact. These groups of cells undergo the same process 

 that the cells themselves do. They increase up to a certain 

 point, and then multiply either by simple spontaneous fission or 

 by that modification of it called gemmation. The Volvox globator, 

 which is made up of a number of monads associated together in 

 the form of a hollow sphere, develops within itself a number of 

 smaller spheres similarly constituted ; and after these, swimming 

 freely in its interior, have reached a certain size, the parent 

 group of animalcules bursts and sets the interior groups free. 

 And here we may observe how this compound individuality of 

 the Volvox is destroyed in the act of reproduction as the simple 

 individuality of the monad is. Again, in the higher forms of 

 grouped cells, where something like organisation begins to show 

 itself, the aggregations are not only larger, but the separative 

 process, now carried on by the method of gemmation, no longer 

 wholly destroys the individual. And in fact, this gemmation 

 may be regarded as the form which spontaneous fission must 

 assume in ceasing to be fatal ; seeing that gemmation essentially 

 consists in the separation, not into halves, but into a larger part 

 and a smaller part ; the larger part continuing to represent the 

 original individual. Thus in the common Hydra or fresh-water 

 polype, " little bud-like processes are developed from the external 

 surface, which are soon observed to resemble the parent in 

 character, possessing a digestive sac, mouth, and tentacula ; for 

 a long time, however, their cavity is connected with that of the 

 parent ; but at last the communication is cut off, and the young 

 polype quits its attachment, and goes in quest of its own main- 

 tenance." \ 



7. Progress from these forms of organisation to still higher 

 * Prin. of Phy*., p. 256. f ^"*> P- 212. \ Ibid., p. 266. 



