[MACALLxrM] SEMI-CENTENNIAL OF THE ORIGIN OF SPECIES 188 



very important determinants of its structure. It is only necessary to 

 bring forward illustrations of the effects of disuse from animals which 

 inhabit caves. These are all either partially or totally blind, for the 

 power of vision is useless in the darkness of cavern life. The eyes are 

 either reduced in structure, partially atrophied or undeveloped. An 

 example of the last condition is found in some of the crabs in which the 

 " footstalk for the eye remains, though the eye is gone, — the stand for 

 the telescope is there, though the telescope with its glasses has been lost "^ 

 (Darwin). For the effects of use Darwin referred to the insects of 

 Madeira which are not ground-feeders, and which as flower-feeders have 

 to battle with the wind so as not to be blown out to sea. In these the 

 wings are fully developed and even enlarged, while in the ground-feeding 

 insects the wings are atrophied because normal wings have been of no 

 service and were, therefore, not used. 



Such results were supposed by Lamarck and his school to be ex- 

 plained by the transmission of acquired characters, to use Weismann's 

 expression, but Darwin nowhere expresses an unqualified adhesion to 

 this view. Indeed in a letter ^ to A. de Candolle he wrote : — " I must 

 own that I have great difficulty in believing that any temporary condi- 

 tion of the parents can affect the offspring. If it last long enough to 

 affect the health or structure of the parents I can quite believe the off- 

 spring would be modified." It is to be noted that a rigid examination 

 of all the cases which seem to prove the direct transmission of acquired 

 characters had not been made before Weismann undertook to examine 

 the record in 1881 and consequently it was incumbent upon Darwin to 

 advance an explanation of how this transmission occurred. Heredity 

 was a known force, and, as it was supposed to be exemplified in the 

 transmission of acquired peculiarities, the result was the production of 

 the theory of Pangenesis. 



This theory briefly stated postulated that every cell of the body 

 gives off small particles of a character peculiar to and representative of 

 each of such cells, and that these circulating in the body fluids, blood and 

 lymph, finally reach and penetrate the sexual cells, the ova and sperma- 

 tozoa. The latter thus contain representative particles from every cell 

 of the body and thus each ovum or spermatic element is, as it were, a. 

 microcosm of the parent body. These representative particles, or, as 

 Darwin called them, " gemmules " act in such a way in the ovum, after 

 fertilization takes place, as to bring about in the offspring thus arising 

 a complete reproduction of all the characters of both parents. The 



1 More letters of Charles Darwin, 1903, Vol. I, p. 348. 

 Sec. IV., 1909. 11. 



