GROWTH IN RESPONSE TO INJURY 9 



for, with very primitive organization, the passage of food from the 

 periphery to the centre through living and assimilating tissue 

 cannot have been great. Very possibly, too, these lowly organisms 

 were so loosely compacted that they easily fell to pieces under 

 shocks from the environment. Every fragment, then, constituted 

 a member of the species. Doubtless definite shape and size, as 

 well as definite differentiation of structure within the individual 

 and particular methods of reproduction which resulted therefrom, 

 such as periodic self-division, were products of prolonged evolution. 

 Probably the lowest living beings which we are capable of distin- 

 guishing as living, even under the highest powers of the microscope, 

 are products, as regards size and in other ways, of ages of evolution. 

 So far, of course, all is pure conjecture ; but it is something more than 

 conjecture that early evolution must have consisted of such changes 

 in the germ-plasm, such changes in the hereditary tendencies, that 

 each species came to respond, by growing in its own particular 

 way, to the stimulus of nutriment, and so evolved its own peculiar 

 characteristics. 



15. We can guess only vaguely at the stage of evolution at 

 which the power of responding by growth to the influence of 

 injury, and so repairing damaged structures, first appeared. If the 

 primitive living beings were capable of indefinite growth, but not 

 of spontaneous self-division, then this continued growth after 

 fragmentation in itself constituted a sort of repair of injury. 

 Probably, therefore, the power of responding to the stimulus of 

 injury was evolved concurrently with that of responding to the 

 stimulus of nutriment and was originally part of and indistinguish- 

 able from it. Even in higher types, unicellular and multicellular, 

 which have evolved heterogeneous structures and are of more 

 or less definite shapes and sizes, the underlying resemblance 

 remains ; nutriment stimulates the individual to produce the 

 characters of the species, injury stimulates him to reproduce them 

 more or less perfectly when they are lost or damaged. 



1 6. This power of reproducing lost or damaged parts, of growing 

 under the stimulus of injury, varies greatly in different species and 

 in different structures of the same species. Since it is a product 

 of evolution, since it is one of the means by which organisms are 

 adapted to the environment, its degree of development in any 

 species or structure is proportionate to its utility. Relatively 

 speaking, the higher animals, which have very complex organiza- 

 tions and lead very active lives, possess it in small measure. As 

 a rule they replace important losses of tissue only by scars. It is 



