122 THE GERM-PLASM 



'lower'' forms, as such, always possess the power of replacing 

 lost parts in a greater degree than do the higher ones. It is 

 true that the capacity for regeneration is certainly much slighter 

 in fishes than in the more highly organised amphibians ; but 

 althousfh the limb of a Triton becomes restored, and the fin of 

 a fish does not, it must not be forgotten that the physiological 

 importance of the two organs is somewhat unequal. On the 

 other hand, the fore limb of a Triton and the arm of a man are 

 not only homologous structures, but are also of almost equal 

 physiological importance, and yet their power of regeneration is 

 very unequal. We must therefore inquire into the causes of this 

 dissimilarity. The power of regeneration in any particular part 

 cannot depend only on the conditions which exist as regards the 

 species under consideration : it must also be due to arrangements 

 for reg^eneration which have been transmitted bv the series of 

 ancestors of this species. Leaving this question aside, and re- 

 garding the power of regeneration as merely depending in each 

 individual case on adaptation, we should arrive at some such 

 conclusion as the following : — the provision of the cells of a 

 certain part with supplementary determinants for the purposes 

 of regeneration, depends primarily on the liability of this part to 

 frequent injury — that is to say, on the degree of probability of 

 injury. Precautions are not taken for injuries which seldom 

 occur, even though these may be very disadvantageous to the 

 individual ; for the loss thereby resulting to the species as regards 

 the number of individuals would be extremely small and unim- 

 portant, and therefore processes of selection would not take 

 place in order to counterbalance this loss. 



In the second place, Xho: physiological or biological importance 

 of the organ itself must be taken into consideration. A useless 

 or almost useless rudimentary part may often be injured or torn 

 off without causing processes of selection to occur which would 

 produce in it a capacity for regeneration. Thus, so far as is 

 known, the minute limbs of Siren and Proteus, which are often 

 bitten off, or not replaced ; while the gills of these animals and 

 of the Axolotl, which are equally liable to similar injuries, become 

 regenerated : — in the latter case the organs are physiologically 

 valuable, while in the former they are not. The tail of a lizard, 

 again, which is very liable to injury, becomes regenerated, be- 

 cause, as we have seen, it is of great importance to the individual, 

 and if lost its owner is placed at a disadvantage. 



