AND MORPHOLOGY OF APHIS. 215 



to commence an independent life. But I ask, how does this explain agamogcnesis ? 

 Why does not the epithelium of tlie ovarium (which is as little or less changed) give rise 

 to young without impregnation ? Why are not the young cells of glands, which are as 

 little changed, "parthenogenetic"? Why, finally, does not the deep substance of our 

 epidermis and epithelium, which al)solutely more nearly resembles embryonic tissue than 

 the structure of the pseudovariiuu does, give rise to young ? 



It maybe replied, however, that the supposed " spermatic force" is exhausted by the re- 

 peated subdivisions of the germ-cell before it becomes a part of the deep epidermic tissue ; 

 for it is one condition of the hj'j^jothesis, that every successive generation or scries of spon- 

 taneous fissions of the prhnary impregnated germ-cell must weaken the " spermatic force" 

 transmitted to such successive generation of cells. 



I presume, however, that the original " spermatic force" is at least as strong in a Man as 

 in an Aphis. The average size of tlie embryo-cells in Aphis is at least not greater than 

 in Man, and the specific gravities of theii' essential tissues are not very different ; so that 

 we may fairly assume that as many embryo-cells go to form a given mass of Aphis as of 

 Man. In that case the impregnated embryo-cell mvist svibdivide as often ; and therefore 

 the " spermatic force " must become as much exhausted in forming, say, a grain or a pound 

 of Aphis, as in giving rise to the like quantity of human substance. 



In his Lectures, Prof. Owen adopts the calculations taken by Morren (as acknowledged 

 by Mm) from Tougard, that a single impregnated ovum of Aphis may give rise, without 

 fecundation, to a quintilliou of Aphides*. I will assume that an Aphis weighs trfeotli of 

 a grain, which is certainly vastly under the mark. A quintilliou of Axihides wUl, on tliis 

 estimate, weigh a qviatrillion of grains. 



He is a very stout man who weighs two million grains ; consequently the tenth brood 

 alone, if all its members siu-vive 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 if the law cited above be correct, the " spermatic force" in each ceU of an 

 Aj)his of this brood must be diminished 500,000,000 times as much as that of a single 

 human cell; nevertheless the "spermatic force" of the Aphis cell is enough to impel it 

 to the production of young, while that of the human cell is not ! 



When to these considerations I add, that it has been shown that the agamic propaga- 

 tion of the Aphis may, under proper conditions, be continued for foui* years Avithout 

 interruption, in which case the "spermatic force" in the later broods must stand in an 

 infinitely minute ratio even to that contained in the cells of the tenth generation, the 

 reductio ad absiirdiim by simple arithmetic, of the so-called explanation, appears to me to 

 be sufficiently obvious. 



For the sake of argument, however, I am willing still to suppose for a moment that 

 agamogenesis does take place in consequence of the retention of a " spermatic force." But 

 I must ask, how does this phi-ase constitute an explanation of the phenomena ? Nothing 

 is more common than the misuse of the word "force" on the part of those who are more 

 versed in the phraseology, than trained in the severe methods, of physical science. The 

 impatient inquirer every now and then calls in the aid of molecular force, or chemical 



* I have not thought it worth while to add, in the products of the generations preceding the tenth. 



