112 THE 1^'DUCT10NS OF BIOLOGY. 



accompanied by great size. The propositioii here illustrated 

 is the converse one, that great size is habitually accompanied 

 by high organization. The conspicuous fact that the largest 

 species of both animals and vegetals belong to the highest 

 classes ; and that throughout their various sub-classes the 

 higher usually contain the more bulky forms ; shows this 

 connexion as clearly as we can expect it to be shown, amid 

 60 many modifying causes and conditions. 



The relation between growth and supply of available 

 nutrimentj is too familiar a relation to need proving. There 

 are, however, some aspects of it that must be contempilated be- 

 fore its implications can be fully appreciated. Among 

 plants, which are all constantly in contact with the gaseous, 

 liquid, and solid matters to be incorporated with their tissues ; 

 and which, in the same locality, receive not very unlike 

 amounts of light and heat ; diiferences in the supplies of 

 available nutriment, have but a subordinate connexion with 

 differences of growth. Though in a cluster of herbs spring- 

 ing up from the seeds let fall by a parent, the greater size of 

 some than of others is doubtless due to better nutrition, 

 consequent on accidental advantages ; yet no such inter- 

 pretation can be given of the contrast in size between these 

 herbs and an adjacent tree. Other conditions here come into 

 play : one of the most important probably being, an absence in 

 the one case, and j)resence in the other, of an ability to se- 

 crete such a quantity of ligneous fibre as will produce a stem 

 capable of supporting a large growth. Among 

 animals, however, which (excepting some Entozoa) differ 

 from plants in this, that instead of bathing their surfaces, 

 the matters they subsist on are dispersed, and have to be 

 obtained ; the relation between available food and growth, 

 is shown with more regularity. The Protozoa, living on 

 microscopic fragments of organic matter contained in I he 

 surrounding water, are unable, during their brief lives, to 

 accvmiulate any considerable quantity of nuti'iment. Polypes 

 and MoUmcoida, having for food these scarcely visible mem- 



