28 GENERAL EVOLUTION. 



witli an excess of growth-force. The type of the preceding gen- 

 eration being strictly adhered to according to the laws of inherit- 

 ance, the only opportunity for its exhibition was necessarily after 

 the inherited parts were completed by acceleration. The results 

 could only, therefore, be exhibited on the edges of the new septa 

 and free margin of the outer wall. As the older growths me- 

 chanically restrained the new septa from mere extension, folding 

 or plication must have resulted from the crowding of additional 

 surface into the small space. This folding would take the form 

 of simple branching, and then " repetition " of the process by a 

 refolding of the outlines of the first fold. The edge would thus 

 have the digitate or pinnately branched appearance it is known to 

 present. 



Another case of folding is that of the brains of the higher 

 mammals, including man. This was occasioned by excess of 

 growth-force distributed to the circumference (from an inherited 

 central region), exceeding in amount that of the space (skull) in 

 which it was inclosed. Folding was necessary for its accommoda- 

 tion in the contracted space. This is a highly useful modification. 

 Another case like that of the ammonite above cited, whose use is 

 not so readily determined, but which illustrates excess of growth- 

 force on some other account, is that of cotyledonary leaves in some 

 plants. In these cases the growth of the leaf is accelerated, re- 

 quiring plication for its accommodation. 



We therefore have the two laws regulating the " location of 

 growth-force " irrespective of use, viz. : 1, the centripetal or po- 

 lar tendency ; 2, the peripheral tendency ; both more or less de- 

 termined by the inherited central or first grown parts. 



There is, however, another element which in animals disturbs 

 the symmetrical direction of growth-force besides use, and which 

 precedes use, i. e., effort. Use presupposes a part to use, and a 

 simple part is sufficient for its influence, so that it be usable ; but 

 the first beginnings of few structures are usable. As Mivart has 

 objected to natural selection, that the first rudiments of organs 

 could never be sufficiently important to an animal to give it a 

 preference in the struggle for existence, so it might be urged 

 against the theory of use that rudiments are generally useless, 

 and could not have been originated by use. 



The effect of use is, however, twofold. The contact with ob- 

 jects used has some effect in stimulating nutrition, as well as the 

 exertion of the muscles necessary to use. But determination of 



