Here, then, we have nine factors, several of them involving subdivisions, which co-operate in aiding or restraining cell-multiplication. They occur in endlessly varied proportions and combinations; so that every species differs more or less from every other in respect of their effects. But in all of them the co-operation is such as eventually arrests that multiplication of cells which causes further growth; continues thereafter to entail slow decrease in cell-multiplication, accompanying decline of vital activities; and eventually brings cell-multiplication to an end. Now a recognized principle of reasoning—the Law of Parsimony—forbids the assumption of more causes than are needful for explanation of phenomena; and since, in all such living aggregates as those above supposed, the causes named inevitably bring about arrest of cellmultiplication, it is illegitimate to ascribe this arrest to some inherent property in the cells. Inadequacy of the other causes must be shown before an inherent property can be rightly assumed.
For this conclusion we find ample justification when we contemplate types of animals which lead lives that do not put such decided restraints on cell-multiplication. First let us take an instance of the extent to which (irrespective of the natures of cells as reproductive or somatic) cell-multiplication may go wHere the conditions render nutrition easy and reduce expenditure to a minimum. I refer to the case of the Aphides. Though it is early in the season (March), the hothouses at Kew have furnished a sufficient number of these to show that twelve of them weigh a grain—a larger number than would be required were they full-sized. Citing Prof. Owen, who adopts the calculations of Tougard to the effect that by agamic multiplication "a single impregnated ovum of Aphis may give rise, without fecundation, to a quintillion of Aphides," Prof. Huxley says:
And had Prof. Huxley taken the actual weight, one twelfth of a grain, the quintillion of Aphides would evidently far outweigh the whole human population of the globe: five billions of tons being the weight as brought out by my own calculation! Of course I do not cite this in proof of the extent to which multipli-
- The Transactions of the Linnæan Society of London, vol. xxii, p. 215. The estimate of Réaumur, cited by Kirby and Spence, is still higher—"In five generations one Aphis may be the progenitor of 5,904,900,000 descendants; and it is supposed that in one year there may be twenty generations" (Introduction to Entomology, vol. i, p. 1 75).