376 PEO VISIONAL HYPOTHESIS Chap. XXVII. 



tion in the sexual elements favours in a marked manner their 

 union and subsequent development, as well as the vigour of 

 the offspring thus produced. 



Thus far we have been able by the aid of our hypothesis 

 to throw some obscure light on the problems which have 

 come before us ; but it must be confessed that many points 

 remain altogether doubtful. Thus it is useless to specu- 

 late at what period of development each unit of the body 

 casts off its gemmules, as the whole subject of the develop- 

 ment of the various tissues is as yet far from clear. We 

 do not know whether the gemmules are merely collected by 

 some unknown means at certain seasons within the reproduc- 

 tive organs, or whether after being thus collected they rapidly 

 multiply there, as the flow of blood to these organs at each 

 breeding season seems to render probable. Nor do we know 

 why the gemmules collect to form buds in certain definite 

 places, leading to the symmetrical growth of trees and corals. 

 We have no means of deciding whether the ordinary wear 

 and tear of the tissues is made good by means of gemmules, 

 or merely by the proliferation of pre-existing cells. If 

 the gemmules are thus consumed, as seems probable from 

 the intimate connection between the repair of waste, re- 

 growth, and development, and more especially from the 

 periodical changes which many male animals undergo in 

 colour and structure, then some light would be thrown on 

 the phenomena of old age, with its lessened power of re- 

 production and of the repair of injuries, and on the obscure 

 subject of longevity. The fact of castrated animals, which 

 do not cast off innumerable gemmules in the act of reproduc- 

 tion, not being longer-lived than perfect males, seems opposed 

 to the belief that gemmules are consumed in the ordinary 

 repair of wasted tissues ; unless indeed the gemmules after 

 being collected in small numbers within the reproductive 

 organs are there largely multiplied. 50 



That the same cells or units may live for a long period and 



50 Prof. Ray Lankester has dis- parative Longevity in Man and the 



cusaed several of the points here Lower Animal;,,' 1870, pp. 3J, 77. 



referred to as bearing on pangenesis, &c. 

 in his interesting essay, ' On Com- 





