INHERITANCE OF ACQUIRED CHARACTERS 187 



light and heat from the sun across an intervening space of almost 

 inconceivable extent which is supposed to contain nothing but 

 the hypothetical and intangible ether. To account for such 

 transmission we postulate the occurrence of vibrations or undula- 

 tions in this ether, and the truth of this theory may now be 

 regarded as conclusively demonstrated. The stimulation of the 

 sensory cells of the retina by certain of these vibrations gives rise, 

 when transmitted along the optic nerve to certain cells in the brain, 

 to the sensation which we recognize as light, while other 

 vibrations are perceived by us as heat. 



The rapid development of wireless telegraphy in recent years 

 has familiarized us with another type of vibration in the ether, 

 by taking advantage of which we are able to send messages over 

 immense distances from one instrument to another, without the 

 aid of wires or any other material connections. The Rontgen 

 rays, again, can make impressions upon sensitive photographic 

 plates after passing through solid objects which are quite imper- 

 vious to the ordinary light vibrations. 



In view of these facts it seems absurd to deny that the living 

 cells of the sorna, in which doubtless complex vibrations, possibly 

 comparable to those which are responsible for the phenomena 

 of light and electricity, are constantly going on, may conceivably 

 influence the germ cells without our being able to demonstrate 

 the existence of material connections by which the necessary 

 stimuli might be transmitted. 



The different means of communication between one cell and 

 another in the living body may be compared to the different 

 methods by which messages are transmitted between distant 

 members of a civilized human community. The circulation of 

 fluids (such as the blood) distributes throughout the body definite 

 substances secreted by certain cells, which may act as stimu- 

 lants upon other and far distant cells. We may compare this 

 to the transmission of messages by letter post. The nervous 

 system of the higher animals, and probably also the delicate threads 

 of protoplasm which so frequently connect one cell with another 

 both in animals and plants, provide a means of communication 

 which is closely comparable to ordinary telegraphy over conducting 

 wires. Why should we deny the possibility that a third means 

 of communication, analogous to wireless telegraphy, may also 

 exist ? 



Darwin's hypothesis of pangenesis depends upon the 



