ON THE PROTOPLASTID BODY, ETC. 331 



ductive centres, may be viewed either as the cause or the 

 consequence of the production of a highly specialised 

 soma, somewhere along the life cycle, a soma which can 

 be cast off and perish, so long as the original cyclical 

 character of the metamorphoses in some elements of 

 the organism is retained. It is shown, however, from 

 the study of plants, that the development of the soma 

 which constitutes their metaplastid individuality, may occur 

 at different points anywhere along the course of the 

 original life cycle ; thus the soma which constitutes the 

 ordinary fern plant is serially homologous with the plant 

 or individual of a lily, but in the life cycle of a lily, after 

 the formation of the sexual cells there is a limited develop- 

 mental period, the elements produced in which correspond 

 to those which build up the soma or plant of a fern pro- 

 thallium and vice versa. Moreover, in some cases, as in the 

 ferns, more than one portion of the life cycle can be 

 developed into a metaplastid plant, and the individuals thus 

 arising may be told off to perform different functions 

 in the economy of the whole cycle, and by their 

 successional development, constitute the alternation of 

 sexual and asexual generations. At the same time, as 

 Huxley (6) pointed out years ago, when treating of the 

 analogous phenomena presented by the various forms of 

 Salps, "It cannot be too carefully borne in mind that 

 zoological individuality is very different from metaphysical 

 individuality, and that the whole question of the propriety 

 of the ' Alternation theory,' as a means of colligating the 

 facts (for at best it can be nothing more), turns upon the 

 nature and amount of this difference ". 



"If the true definition of the zoological individual be 

 (as the writer believes it to be) the sum of the phenomena 

 successively manifested by, and proceeding from, a single 

 ovum, whether these phenomena be invariably collected in 

 one point of space or distributed over many, then there is 

 no essential difference between the reproductive process in 

 the higher and lower animals, and the alternation theory 

 becomes unnecessary." 



From all this, it would appear then, that the formation 



23 



