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



effects of close interbreeding in reducing vitality are too well 

 known to require farther notice. 



Only in exceptional instances can individual pathologic con- 

 ditions have any effect on a stock. The retrogressive series of 

 animals which are diseased in appearance, and are considered 

 by Myatt" as akin to pathologic distortions, are apparently 

 types which have ceased to advance physiologically, and are 

 therefore only adapted to special sets of conditions. In these, 

 the pathologic or abnormal condition is racial instead of indi- 

 vidual, and its cause seems to be a deficiency of vital power 

 combined with great external differentiation, the final result 

 being the assumption of characters belonging to second child- 

 hood and ending in extreme senility, with the loss of spines 

 and other ornaments. 



The life history of parasitic organisms shows their origin 

 from higher normal types by a process of retrogression through 

 loss of motion and disuse of parts. Their mode of living 

 implies dependence upon the vitality of an immediate host, 

 and altogether they may be deficient in the energy of growth 

 force. 



Any of the preceding factors, single or combined, acting 

 upon an organism or group of organisms will produce suppres- 

 sion of structures or functions. Whether from external or 

 internal causes, the waning and disappearance of characters are 

 almost always inversely to the order of their development or 

 appearance, either in the race or in the individual, and the 

 most primitive or axial characters are therefore the most per- 

 sistent and the last to disappear. In this way, a leaf may be 

 suppressed into a spine representing the midrib, a branch into 

 a spiniform twig, a leg or digit into a spine, etc. As in other 

 primary causes of spine genesis, there may also come sec- 

 ondary influences of protection, offense, etc., controlled by 

 natural selection. 



It will be convenient to consider spine production from lack 

 of growth force under three heads : 



D. Deficiency of Growth Force. 



D I. — Intrinsic suppression of structures and functions. 



D 2.— Disuse. 



D3. — Secondarily for protection, etc. 



[To be continued.] 



