THE BEGINNINGS OF IIFE. 565 



These processes of ^ alternate generation ^ formerly 

 attracted very much attention, and were first pro- 

 minently brought under the notice of naturalists by 

 Steenstrup, in 1842 ^ Whether occurring amongst 

 animal or vegetal forms, the processes are essentially 

 characterized by the fact that a fertilized germ goes 

 through two or more metamorphic stages before at- 

 taining the perfect form in which similar fertilized 

 germs are evolved ; whilst in each of the lower stages 

 the immature forms possess the power of multiplying 

 agamically= By common consent such a succession of 

 forms is admitted to correspond to what is known as 

 an organic ^species.' The fact of the metamorphosis 

 could of course have no effect in negativing such a 

 view, seeing that it has hitherto afforded no obstacle 

 in the case of insects : although in instances of ^ alter- 

 nate generation,' a further complication, it is true, is 

 introduced by the fact of the multiplication which takes 

 place amongst the lower metamorphic forms, and by the 

 fact that the successive stages are occasionally mere 

 derivative forms produced by a process of ^gemmation/ 

 The fact of the multiplication of the transition forms, 

 however, cannot introduce any real difficulty, since a 



heterogenetic germ is large, the organism develops without metamor- 

 phosis, and subsequently produces large germs or ova, which also go 

 through a similarly ' direct ' process of development. Instances of this 

 occur amongst Nematoids and Rotifers, and probably also amongst 

 some Medusae, Trematodes, and other low forms of life. 



1 In his ' Generations- Wechsel,' of which a translation was published 

 by the Ray Society in 1845. 



