640 APPENDIX B. 



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 r 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 cell-multiplication, it is illegiti 

 mate 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 con 

 template 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 natures of cells as 

 reproductive or somatic) cell-multiplication may go, where the 

 conditions render nutrition easy and reduce expenditure to a 

 minimum. 1 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 Professor Owen, who adopts the calculations of 

 Tougard to the effect that by agamic multiplication &quot; a single 

 impregnated ovum of Aphis may give rise, without fecundation, 

 to a quintillion of Aphides,&quot; Professor Huxley says : 



&quot; I will assume that an Aphis weighs -n^ of a grain, which is certainly 

 vastly under the mark. A quintillion of Aphides will, on this estimate, 

 weigh a quatrillion of grains. He is a very stout man who weighs two million 

 grains; consequently the tenth brood alone, if all its members survive 

 the perils to which they are exposed, contains more substance than 500,- 

 000,000 stout men to say the least, more than the whole population of 

 China ! &quot; * 



And had Professor Huxley taken the actual weight, one-twelfth 

 of a grain, the quintillion of Aphides would evidently far out 

 weigh the whole human population of the globe : five billions of 

 tons being the weight, as brought out by my own calculation ! Of 



* The Transactions of the Linnrean Society of London, Vol. XXII, p. 215. 

 The estimate of Reaumur, cited by Kirby and Spcnce, is still higher &quot; in five 

 generations one Aphis may be the progenitor of 5,904,900,000 descendants ; 

 and that it is supposed that in one year there may be twenty generations.&quot; 

 (Introduction to Entomology, Vol. I, p. 175) 



