ORIGIN AND SIGNIFICANCE OF SPINES 35 



In multicellular organisms the growth force is in both germ 

 and soma cells, and its relative strength seems to depend 

 upon its power to reproduce lost parts, often including germ 

 cells as well as soma cells. In many of the lower classes 

 the growth force is able to complete a structure or lost part 

 without the stimulus of use, which in higher animals often 

 seems to be part of the necessary requirements for growth. 



Growth itself is the repetition of cells under nutrition and 

 stimulus, and the latter may be hereditary or extra-indi- 

 vidual. It is now recognized that since the division of a cell 

 makes two unlike cells, each unlike the parent, such repeti- 

 tion will produce structures which present some degree of 

 difference. The variation is therefore a necessary quality 

 of growth, and its degree will change in response to the 

 differentiation of the forces affecting growth. 



When spines which have arisen from intrinsic growth force 

 only are sought, it is apparent that they cannot be distin- 

 guished from those arising from external stimuli acting on 

 and directing the growth force, unless in some instances 

 they are found to be developed independently or even at 

 variance with the environment. Because spines are some- 

 times useful to the organism, it is impossible to believe that 

 they have originated from that cause, since their existence in 

 some form must precede the capacity of making them useful. 

 After they began to develop by either intrinsic or extrinsic 

 forces, their being found useful would simply tend to their 

 conservation and further development. 



Variation which is not restricted by natural selection or a 

 long line of hereditary tendencies is known as free variation. 

 It is best exhibited in a stock which occupies for a consider- 

 able time a region favorable in respect to food, climate, and 

 absence of dominating natural enemies. This relation has 

 been called the period of " Zoic maxima " by Gratacap, 23 

 and has been further discussed by the same author, under 

 the aspect of numerical intensity. 22 The most rapid rise of 

 a stock is considered to be consequent to a favorable environ- 

 ment and high vitality. 



