394 THE EVOLUTION THEORY 



stituonts of all cell-groups as so indefinite. Let us call to mind, for 

 instance, the venation of the insect wing. It is well known that this 

 is not only ([uite different in beetles, bugs, and Diptera from that in 

 tlu' Ihnienoptera, and diti'erent again in the butterflies, but that it is 

 • [uite characteristic in every individual faniil}^ of butterflies, and 

 indeed in every genus. We cannot conceive of the absolute certainty 

 of development of these very characteristic and constant branchings as 

 having its roots elsewhere than in the determinants of the germ-plasm, 

 which, lying within certain series of cells, ultimately cause particular 

 cell-series of the wing-rudiment to become the wing- veins. If this 

 were not so, how would it be possible to understand the fact that 

 every minute deviation in the course of these veins is repeated in 

 exactly the same way in all the individuals of a genus, while in all 

 the individuals of an allied genus the venation turns out slightly 

 different with equal constancy. 



But it is quite certain that all determinations are in some degree "^ 

 .susceptible to modifjdng influences, that they are in very different / 

 degrees capable of variation. 



Many deformities of particular parts in Man and the higher 

 animals may be referred to imperfect or inhibited nutrition of the part 

 in question during embryonic development ; the determinants alone 

 cannot make the part, they must have a supply of formative material, 

 and according as this material is afforded more abundantly or more 

 scantily the part will turn out larger or smaller. In the same way 

 the pressure conditions of the surrounding parts must in many cases 

 have a furthering or inhibiting influence, or may even determine 

 the shape. But it is quite possible, indeed even probable, that other 

 specific influences are exerted by the cells or cell-aggregates sur- 

 rounding an organ which is in process of being formed, just as the 

 stake on which a twining plant is growing may prompt it to coil. If 

 the stake be absent, the predetermined twining of the plant cannot 

 attain to more than very imperfect expression, if indeed it finds any. 

 The spirally coiled sheath of muscle-cells which occurs so often around 

 blood-vessels in worms, Echinoderms, and Vertebrates is probabl}' due 

 to similar processes, that is, on the one hand, to a specific mode of 

 reaction characteristic of these cells, and predetermined from the germ ; 

 on the other hand, to the external influence of the cell-surroundings 

 without which the determination of the muscle-cell is not liberated, 

 that is, is not excited to activity. 



But even if every determinant requires a stimulus to liberate it,f 

 whether this stimulus consists in currents of particular nutritive fluids, . 

 in contact with other cells, or, conversely, on the removal of some! 



