703 



duces its own likeness, but more careful observation has shown that this 

 is far from being uniformly the case, or in other words, that the law^ 

 of reproduction is irregular unless it be in undetermined cycles. 

 Every one knows that the entomologist has laid down a law for the 

 reproduction of insects; he tells us that each individual commences life 

 as an egg, then becomes a larva, then a pupa, and lastly an imago; but, 

 the little insignificant plant-louse refuses these conditions, and mounts 

 upwards one step with one or several generations ; thus the egg he- 

 comes a larva, the larva produces a larva, this larva a pupa, and the 

 pupa becomes an imago. In the ' Entomological Magazine' for April, 

 1833, was published the astounding and then incredible fact that the 

 hop-fly was sometimes born with wings.* But this discovery sinks into 

 insignificance compared with observations lately published by a Ger- 

 man naturalist, on some of the more obscure tribes of radiate animals. 

 This author asserts that certain forms which have long been considered 

 perfect animals, and have been ranged in our methods under other fixed 

 technical names, are absolutely the parents or progenitors of certain 

 other animals, which are also ranged in our methods under other fixed 

 technical names : and it seems that these nurses may succeed each 

 other, as in the Aphides, for many generations. In introducing this 

 brief notice of Steenstrup's work, I do not wish to be understood as 

 adopting his views ; all I can venture to say is, that his observations 

 appear to have been made with patience and care, and his deductions 

 drawn with perfect candour and fair reasoning, so that his work is en- 

 titled to our respect. Seeing, then, how difficult it is to fix on states 

 that shall exactly correspond, a comparison of the radiates and thallo- 

 gens must be madefrom a general, rather than an individual, similarity: 

 and we shall find in their equivocal mode of reproduction, in the 

 obscurity, if not absence of sexes, in the simplicity of their structure, 

 and in their general tendency to radiation from a centre, that there is a 

 general similarity between these two primary divisions. 



The acrogens, restricted to the ferns, mosses, &c., are supposed by 

 systematists to be the analogues of the mollusks, including the Anneli- 

 des ; and as in the thallogens, so in the acrogens, we have great 

 difficulty in fixing the portion of the plant which shall be the subject 

 of comparison. The frond of a fern has been called a branch, a leaf, 

 and a flower ; but the general plan seems to be to treat the fronds, taken 

 collectively, as the entire plant : now if this view be correct, I confess 

 that there is but slender hope of establishing any analogy between a fern 

 and a great yellow slug, or any of the slug tribe. But let us pause 



*Ent. Masr., i. 315. 



