4 C. E. Beecher — Origin and Significance of Spines. 



factors of evolution, while regressive variation is towards disso- 

 lution. Since the main history of life is told through processes 

 of the former, progressive variation is far greater in importance ; 

 while, in general, regressive variation can be applied only to 

 late periods in the history of groups or forms now in their 

 decadence, or to others which in past times have suffered 

 decline and extinction. 



The summary of the operation of the law of multiplication 

 of effects, as given by Herbert Spencer 60 , may well be stated 

 here, as it emphasizes one of the principles through which 

 spines have originated. 



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

 form aggregate, must undergo dispersion ; that falling on an 

 aggregate made up of unlike parts, it must undergo dispersion 

 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 evolution 

 is the multiplication of effects, and that this increases in geo- 

 metrical 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 knowledge 

 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 without 

 limit or is it restricted within bounds which can be deter- 

 mined? 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 restrictions of varia- 

 tion are chiefly those of temperature, pressure, motion, light, 

 space, time, and matter. Within certain limits, these clearly 

 bound the horizon of known possible life. Further, the mate- 

 rial constitution of the organic world is naturally subject to 

 ordinary mechanical and chemical laws. 



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

 abstract statements of the limits of variation, the subject is 



