480 THE POPULAR SCIENCE MONTHLY. 



Here, then, we have nine factors, several of them involving 

 subdivisions, which co-operate in aiding or restraining cell-multi- 

 plication. They occur in endlessly varied proportions and com- 

 binations ; so that every species differs more or less from every 

 other in respect of their effects. But in all of them the co-opera- 

 tion 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 rec- 

 ognized 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 illegitimate to ascribe this arrest to some in- 

 herent 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 in- 

 stance 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 suf- 

 ficient 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 : 



"I will assume that an Aphis weighs Y^Vo of a grain, which is certainly vastly 

 under the mark. A quintillion of Aphides will, on this estimate, weigh a quatril- 

 lion of grains. He is a very stout man who weighs two million grains ; conse- 

 quently 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! "* 



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 Linnaean Society of London, vol. xxii, p. 215. The estimate 

 of Reaumur, 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. 175). 



