6 INTRODUCTORY LECTURE. 



added to self-motion. But growth by elongation and bisection of the 

 cells is very frequent if not universal in the more simple algae ; 

 w^hilst propagation by spontaneous fission is the common mode of 

 increase of the single-celled plants which constitute yeast ( Torula 

 cerevisii) and the colouring matter of red snow (Protococcus nivalis). 

 All living organs are continually receiving additions to their sub- 

 stances ; and so long as these exceed in quantity the parts removed, 

 they are said " to grow." Plants seem to grow as long as they live ; but 

 if the tree and other compound kinds be rightly regarded as organic 

 associations of individual phytons,then these, when fully developed into 

 the form of the leaf, the sepal, the petal, or the pistil, abide, like the 

 polype of the compound coral, without further growth, until the period 

 of their decay or fall. And if this philosophic interpretation of the 

 "tree " be rejected and the common notion of its individuality be held 

 to be the truer one, it will as little assist in differentiating the plant and 

 the animal by the character of growth : for the compound zoophyte must 

 then be regarded as a many-mouthed individual, and any climacteric 

 cessation of its growth can be as- little predicated of it, as of the com- 

 pound individual plant. But in regard to both trees and zoophytes 

 it is plain that there is a certain term of growth, even for the com- 

 pound whole ; for most species have a characteristic maximum of 

 size, otherwise we should not see the Elm of 50 years surpassing the 

 Yew of 500 years : and no term of life would bring the Elm or the 

 Yew to an equality of size with the mighty Boabdad (Adansonia). 



The creatures that appear least equivocally to enjoy gi'owth during 

 the whole term of their existence are the Trout, the Pike, the Afta- 

 cofida, the Testudo elephantopus, and the like cold-blooded vertebrate 

 individuals : although as the rate of growth, which is always slow, 

 becomes slower as age advances, they seem, during the few years 

 that a naturalist can watch an aged individual of these long-lived 

 species, to be stationary in regard to their growth. The records of 

 unusually large specimens of such cold-blooded animals always asso- 

 ciate such uncommon size with far advanced age, and imply that there 

 is no definite period arresting their growth as in the warm-blooded 

 vertebrates. Still, however, as in trees and zoophytes, the recog- 

 nised average size characteristic of different species of fishes and 

 reptiles, which all start from a germ-cell of nearly the same minute- 

 ness, shows plainly that there is a specific limit of growth for each. 

 And this at least is certain, that the " continually receiving additions 

 during the term of existence",* however understood, helps us no way 

 towards the discrimination between plants and animals. 



* V. p. 182. 



