EVOLUTION OF THE COLORS OF BIRDS. 9 



same manner, and finally also divides into two halves. 

 As far as these organisms are concerned, death can only 

 be spoken of in the most figurative sense."* From this 

 view of Weismann's in regard to the immortality of pro- 

 tozoa there has been but 1-ittle dissent. The most seri- 

 ous objection that has been raised to it is that of Maupas, 

 who has shown that even among amoeba conjunction is 

 occasionally necessary to effect rejuvenescence, and who 

 holds that, in consequence of this, even unicellular or- 

 ganisms are mortal. Geddes and Thomson have well 

 stated,! however, that Maupas' experiments, instead of 

 being contradictory to Weismann's view, should be in- 

 serted as a saving clause, for in a state of nature this 

 rejuvenescence by coalescence does take place when 

 needed and neither the organism as a whole nor any 

 part of it dies. 



Having established the immortality of unicellular or- 

 ganisms, Weismann attempts to account for the intro- 

 duction of death into the economy of nature on the 

 principle of the advantage to the race as a whole, of the 

 sacrifice of the old and decrepit to the young and vig- 

 orous. The weaknesses and fallacies of this part of his 

 theory have been pointed out with especial force by Lloyd 

 Morgan, t 



The fact seems to be tolerablj^ well established that in 

 protozoa death never normally ends the career of the 

 organism, although of course violent death by accident 

 is perfectly possible, while in metazoa death is the nat- 

 ural outcome. Or, as Weismann explains it, there is no 

 limit to the number of times an amoeban cell can divide 

 itself, but the cells of a more complex organism are lim- 



* Essays upon Heredity, 1891, I, p. 26. 

 tEvohitioii of Sex, Humboldt Library, p. 240. 

 JAnimal Life and Intelligence, pp. 184 and 193. 



