592 APPENDIX A. 



case. A fertilised germ appears here, as amongst higher organ- 

 isms, to be the point of departure ; and that constant formation 

 of new tissue implied in the production of a great number of 

 individuals by fission, seems gradually to exhaust the germinal 

 capacity in the same way that the constant formation of new 

 tissue, during the development of a single mammal, exhausts it. 

 The phenomena classified by Steenstrup as "Alternate Genera- 

 tion," and since generalised by Professor Owen in his work " On 

 Parthenogenesis," illustrate this. The egg of a Medusa (jelly- 

 fish) develops into a polypoid animal called the Strobila. This 

 Strohila lives as the polype does, and, like it, multiplies rapidly 

 by gemmation. After a great number of individuals has been 

 thus produced, and when, as we must suppose, the germinal 

 capacity is approaching exhaustion, each Strobila begins to exhibit 

 a series of constrictions, giving it some resemblance to a rouleau 

 of coin or a pile of saucers. These constrictions deepen ; the 

 segments gradually develop tentacula; the terminal segment 

 finally separates itself, and swims away in the form of a young 

 Medusa ; the other segments, in succession, do the same ; and 

 from the eggs which these Medusce produce, other like series of 

 polypoid animals, multiplying by gemmation, originate. In the 

 compound Polypes, in the Tunicata, in the Trematoda, and in the 

 Aphis, we find repeated, under various modifications, the same 

 phenomenon. 



Understanding then, this lowest and most rapid mode of 

 multiplication to consist essentially in the production of a great 

 number of individuals from a single germ — perceiving, further, 

 that diminished activity of this mode of multiplication consists 

 essentially in the aggregation of the germ -product into larger 

 masses — and seeing, lastly, that the disappearance of this mode 

 of multiplication consists essentially in the aggregation of the 

 germ-product into one mass — we shall be in a position to com- 

 prehend, amongst the higher animals, that new aspect of the law, 

 under which increased individuation still involves diminished 

 reproduction. Progressing from those lowest forms of life in 

 which a single ovum originates countless organisms, through 

 the successive stages in which the number of organisms so 

 originated becomes smaller and smaller ; and finally arriving 

 at a stage in which one ovum produces but one organism ; we 

 have now, in our further ascent, to observe the modified mode 

 in which this same necessary antagonism between the ability 

 to multiply, and the ability to preserve individual life, is ex- 

 hibited. 



§ II. Throughout both the animal and vegetable kingdoms, 

 generation is effected " by the union of the contents of a 



