THE CHARACTERS OF LIVING BEINGS 



only within narrow limits which are not the same for every species. 

 Some organisms develop markedly diverse characters at different 

 temperatures. Light is essential to the development of very many 

 species, especially of plants. Certain plants develop in one way 

 in the water, and in another way on the land. A brine shrimp 

 (Artemia) develops one form in brackish water and another in 

 water that is salter. Within the animal body the ' internal ' 

 secretion of various glands exercises an important influence on the 

 development of many organs. For the purposes of our discussion, 

 however, all these stimuli, and others besides, may be bracketed 

 with nutriment, use, or injury ; they are, in effect, in many cases 

 at least, conditions under which nutriment and use act as stimuli, 

 or which, when in excess, produce injury. 



13. All the characters which it is possible for an individual 

 under any circumstances to possess are traceable ultimately to an 

 inter-action between the environment and hereditary tendencies or 

 potentialities which have their roots in the germ-plasm. Because 

 the germ-plasm of a dog differs from that of a man, because the 

 hereditary tendencies are different, the two animals develop into 

 very different individuals, and would so develop though placed 

 under identical conditions as regards nutriment, use, and injury. 

 On the other hand, if, were it possible, two human germs were 

 exactly alike in all their potentialities, differences of nutriment, 

 use, and injury would cause corresponding differences between the 

 individuals that arise from them. The human arm first develops 

 under the stimulus of the nutriment that reaches it from the world 

 external to it, because of an antecedent potentiality in the germ- 

 plasm. For the same reason it continues to grow under the in- 

 fluence of use, or, when it is damaged, under the stimulus of injury. 

 Stimulus is the invariable antecedent to all growth. 



14. Since no character can be used or injured until it exists, 

 the power of growing under the stimulus of nutriment must have 

 been the very first product of evolution, the first peculiarity by 

 which living beings were distinguished from mere chemical com- 

 pounds. Possibly the earliest living beings were homogeneous 

 throughout, consisting altogether of a primitive kind of germ-plasm, 

 there being no cell-body as distinguished from the nucleus, no 

 ' cyto-plasm ' properly so called, in which case the minutest living 

 fragment was perhaps capable of continuing the species. Probably 

 also each organism was capable of indefinite growth, or at least 

 of an amount of growth which was limited only by the supply of 

 nutriment. The individual, however, can never have been large ; 



