192 GENERAL EVOLUTION. 



only permits us to believe that other force is only converted into 

 it under the influence of pre-existent life, but of the real cause of 

 this conversion we are as ignorant as in the case of the physical 

 forces. 



In the animal organism, different tissues display different de- 

 grees of " vitality." The most vital display cell-organization and 

 its derivative forms, while the least so, approach nearer to homo- 

 geneity. As organized tissue is the machine for converting vital 

 forces, we may believe that less growth-force is potential as such in 

 cartilage than in muscle, for it is formed by a retrograde process, 

 by which cells once formed are mostly lost, and the contents form 

 the intercellular, nearly structureless mass characteristic of this 

 tissue. Growth-force must be here liberated in some other form, 

 perhaps the mere cohesive force of the former or "dead" inter- 

 cellular substance. 



The higher vitality we may believe to result from the greater 

 perfection of the more complex macliine as a force converter, as 

 compared with the inefficiency of the more simple. 



E. On the Direction of Repetition. 



It has been already pointed out that growth-force exhibits itself 

 in cell or segment repetition. The forms in which it thus displays 

 itself may be briefly considered. The approximate cause is treated 

 of in the next chapter ; but enough may be shown here to indicate 

 that duplication and complex duplication is the law of growth-force, 

 and that therefore this process must always follow an increase in 

 amount in any given locality. 



The size of a part is then dependent on the amount of cell-divis- 

 ion or growth-force, which has given it origin, and the number 

 and shape of segments is due to the same cause. The whole ques- 

 tion, then, of the creation of animal and vegetable types is reduced 

 to one of the amount and location of gr oivth force. 



Kepetition is of two kinds, centrifugal and longitudinal. As 

 an example of the former, the genus Actinophrys has been cited, 

 where the animal is composed of cells arranged equidistally around 

 a common center. The arrangement in this type may be dis- 

 coidal or globular, providing no definite axis be discoverable. As 

 an example of longitudinal repetition, Vibrio, and numerous 

 cellular plants may be cited, where the arrangement is in a single 

 line. 



In by far the greater number of animals these kinds of repeti- 



