REVIEWS— THE GENETIC CYCLE IN ORGANIC NATURE. 5l7 



become the means of its being developed into a body eventually 

 resembling that from which it was itself derived." 



This is cautiously expressed so as to include all the varieties ever 

 supposed of parental derivation, whilst entirely excluding what has 

 been called equivocal generation ; and the paragraph is immediately 

 followed by a candid statement of difficulties raised on that subject, 

 tending to justify the opinion generally prevalent among physiologists 

 unfavourable to the possibility of the origination of organic beings 

 de novo. A large portion of the volume is occupied by a survey of 

 the reproductive system, first in the vegetable then in the animal 

 kingdom. This portion of the work is very valuable to a student, 

 and appears to be a careful summary of ascertained facts, although in 

 some instances already, in the short interval since this account was 

 prepared, the field of knowledge has been enlarged. Since the 

 publication of the volumes of Agassiz on the Acalephae, we can 

 hardly accept as satisfactory the account here given of the Hydrozoa, 

 and other points are more or less questionable, yet we could refer to 

 no better abstract of information on the subject, especially in so 

 accessible a form. 



The chapter on " the Nature and Varieties of Alternation of 

 Generations," is both remarkably interesting in itself and important 

 in its bearing on the author's theoretic views. We make a somewhat 

 extended extract in order to bring the latter before our readers — whilst 

 for the facts we refer them to his own pages. 



§ 1. The two modes of propagation — by gemmae capable of spontaneous 

 evolution, and by germs dependent on impregnation — as has been alreadj 

 observed, are frequently associated with no less remarkable diversities in the 

 immediate result of the development, leading in cases of periodic recurrence 

 or alternation of the former, to a corresponding mutation or alternation of 

 dissimilar forms in the same species. It is only, however, quite recently that 

 this has been admitted generally by zoologists, who were not unnaturally 

 indisposed to it, by observing the constant succession of like to like in the 

 higher animals. But since the time that Chamisso called the attention of 

 naturalists to the recurrence of two forms in Salpa^ as a case of " Alternation 

 of Generations," analogous phenomena have been abundantly brought forward 

 in other tribes of organized beings. Steenstrup was the first to group together 

 these cases, applying to them the same term as was used by the former 

 naturalist, for which some later writers would substitute that of Metagenent, 

 proposed originally by Professor Owen. 



In all these cases we may admit so much as this in common — that an act of 

 4igenesis recurs with greater regularity in the interval of the acts of monogenesis ; 



