ORIGIN AND SIGNIFICANCE OF SPINES 7 



"It manifestly follows that a uniform force, falling on a 

 uniform aggregate, must undergo dispersion ; that falling on 

 an aggregate made up of unlike parts, it must undergo dis- 

 persion from each part, as well as qualitative differentiations; 

 that in proportion as the parts are unlike, these qualitative 

 differentiations must be marked; that in proportion to the 

 number of the parts, they must be numerous; that the 

 secondary forces so produced must undergo further trans- 

 formations while working equivalent transformations in the 

 parts that change them ; and similarly with the forces they 

 generate. Thus the conclusions that a part-cause of evolu- 

 tion is the multiplication of effects, and that this increases in 

 geometrical progression as the heterogeneity becomes greater, 

 are not only to be established inductively, but are deducible 

 from the deepest of all truths." 



Modification, therefore, may properly include the results of 

 the multiplication of effects. Furthermore, from a knowl- 

 edge of the life history of the organic world, it is known that 

 this change has been progressive, resulting in the evolution 

 of the higher from the lower, of the complex from the simple, 

 and of the definite from the indefinite. 



It must now be asked. Is the amount of variation with- 

 out limit or is it restricted within bounds which can be 

 determined? As far as can be seen, the limitations of the 

 forms of species of animals and plants end only with the 

 aggregate number of possibilities within the functional scope 

 of the organism. Beyond, in either direction, is death, and 

 a passage from the organic into the inorganic. The restric- 

 tions of variation are chiefly those of temperature, pres- 

 sure, motion, light, space, time, and matter. Within certain 

 limits, these clearly bound the horizon of known possible 

 life. Further, the material constitution of the organic 

 world is naturally subject to ordinary mechanical and chemi- 

 cal laws. 



If, instead of the preceding, general, and therefore rather 

 abstract statements of the limits of variation, the subject 

 is considered from the concrete, objective side, the limits 



