SPECIALITIES OP THESE RELATIONS. 491 



with a very low rate of consumption and a correlative degra- 

 dation of structure; we have here a retrogression to asexual 

 genesis, and a greatly-increased rate of multiplication. 



The recently discovered instance of internal metagenesis 

 in the maggots of certain Flies has a like meaning. In- 

 credible as it at first seemed to naturalists, it is now proved 

 that the Cecydomia-larva develops in its interior a brood of 

 larvae of like structure with itself. In this case, as in the 

 last, abundant food is combined with low expenditure. These 

 larvae are found in such habitats as the refuse of beet-root- 

 sugar, factories — masses of nitrogenous debris remaining after 

 the extraction of the saccharine matter. Each larva has a 

 practically-unlimited supply of sustenance imbedding it on 

 all sides.* 



It is true that some other maggots, as those of the Flesh- 

 fly, are similarly, or still better, circumstanced; and, it may 

 be said, ought therefore to have the same habit. But this 

 does not necessarily follow. Survival of the fittest will 

 determine whether such specially-favourable conditions result 

 in aggrandizement of the individual or in multiplication 

 of the race. And in the case of the Flesh-fly there is a 

 reason why greater individuation rather than more rapid 

 genesis will occur. For a decomposing animal body lasts so 

 short a time, that were Flesh-fly larvae to multiply agamically, 

 the second generation would die from the disappearance of 

 their food. Hence individuals in which the excessive 

 nutrition led to internal metagenesis, would leave no pos- 

 terity, and natural selection would establish the variety in 



* I am told that " Wagner, who described the larva, found that it bored 

 into the bark of trees. It attacks also the wheat plant, and is a most 

 destructive parasite." Apparently this statement is at variance with the 

 foregoing inference. It is clear, however, that since these heaps of nitro- 

 genous refuse in which it has been found are artificial and recent, they can- 

 not be its natural habitats ; and it seems not improbable that these larvae, 

 suddenly supplied with a more nutritive food in unlimited amount, may have 

 as a consequence acquired this habit of agamogenetic multiplication which 

 did not characterize the species under its natural conditions and relatively low 

 nutrition. 



