12 THE EVOLUTION THEORY 



in means of defence, and possibly also of defiance, as is indeed in some 

 measure demonstrable. 



We meet with the same thing in a group of still smaller worms, 

 Rosel's ' water-snakelets,' species of the genus JWdt^. These, too, 

 behave in a variety of ways in the matter of regeneration, for 

 while many species, such as JS^ais prohoscidea and Nais terpentina 

 will, if cut into two or three pieces, become two or three worms 

 respectively, Bonnet expressly mentions an unnamed species of Nais 

 which does not bear cutting up at all, and even dies if its head 

 be cut off. 



Thus neither the degree of organization nor the relationship 

 alone determines the strength of the regenerative capacity. And 

 as nearly related species may behave quite differently in this respect, 

 so also do the different parts of one and the same animal ; and here, 

 too, the strength of the capacity seems to depend on the more frequent 

 or rarer injury of the relevant part and on its importance in the 

 maintenance of life. Let us take a few examples. 



Parts which, in the natural life of the animal, are never injured, 

 show in many cases no power of regeneration. This is so in regard 

 to the internal parts of the newt, whose regenerative capacity is 

 otherwise so high. I cut half or nearly the whole of a lung away 

 from newts anaesthetized with ether; the wound closed, hut no 

 reneiucd of the organ took place. The same thing happened when 

 a piece of the spermatic duct or of the oviduct was cut away. It 

 is true that the kidney enlarges in higher animals when a piece has 

 been cut out, by the proliferation of the remaining tissues, but that is 

 a mere physiological substitution, evoked by the increased functional 

 stimulus, due to the accumulation in the blood of the constituents 

 of the urine. Such substitution depends on the growth of parts 

 already existing, and it occurs in man when one kidney is removed, 

 for the other, as is well known, may then grow to double its normal 

 size. This is mere hypertrophy of the part that is left, it is not 

 regeneration in the morphological sense, and it is not comparable 

 to the re-formation of a cut-off leg in the salamander, or of a head in 

 the w^orm, where the growth is not a mere increase of the remaining 

 stump, but a new formation. It would be regeneration if a new 

 kidney developed from the remnants of the kidney-tissue, or, in 

 the liver, if new lobes grew in place of those which were cut off. 

 But neither of these things happens, and, as far as I am aware, 

 nothnig of the kind has ever been observed, nothing more than 

 new formation of liver-cells through increase of existing ones ; that, 

 however, is not regeneration in the morphological sense. 



