132 C. K Beecher — Origin and Significance of Spines. 



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. 



Grow th itself is the repetition of cells under nutrition and 

 stimulus, and the latter may be hereditary or extra-individual. 

 It is now recognized that since the division of a cell makes 

 two unlike cells, each unlike the parent, such repetition 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 dis- 

 tinguished 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 environme-nt. Because spines are sometimes 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. 



Yariation 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 considerable 

 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 con- 

 sidered to be consequent to a favorable environment and high 

 vitality. 



In illustration of these points, the Achatinellse of the Sand- 

 wich Islands afford a good example. The great number of 

 species on these islands has probably been evolved since Ter- 

 tiary times, and the process of specific delimitation is appar- 

 ently still going on, for species are now to be found which did 

 not exist fifty years ago (Verrill) ; also, a few species formerly 

 common are now obsolescent or extinct. According to Hyatt, 

 they all can be deducible from a single species which has dif- 

 ferentiated in time through divergence, dispersion, and colonial 

 isolation. In early times, birds may have fed upon them, but 

 the complete or partial extinction of the former by man has 

 resulted in complete immunity for the arboreal Achatinellse, 



