The Origin of Species 325 



genes " is more likely to approximate the truth. 

 These ultimate structural units of the cell are sup- 

 posed to be of relatively few kinds, but capable of 

 an infinite variety of combinations. De Vries com- 

 pares them to the letters of the alphabet, which are 

 capable of combination into an almost infinite num- 

 ber of words. It may be questioned, however, 

 whether specific differences necessarily involve new 

 kinds of pangenes, as De Vries believes to be the 

 case. 



Heredity a Physiological as well as a Morpho- 

 logical Question. The investigations of experi- 

 mental morphology are making it more and more 

 likely that the effects of extrinsic stimuli are potent 

 in heredity, and that heredity is quite as much a 

 physiological problem as a morphological one. This 

 view has been set forth in a particularly striking 

 fashion by Professor F. Darwin, in his recent ad- 

 dress as president of the British Association for the 

 Advancement of Science. Professor Darwin argues 

 that the effects of stimulation may be cumulative, 

 and transmissible, and that the ordered sequence in 

 the development of the individual, " the rhythm of 

 ontogeny," is, as he puts it, a habit. It has been 

 shown by many experiments, e.g., those of Jennings 

 on the infusoria, that the effect of repeated stimula- 

 tion is a different reaction on the part of the cell 

 to the later stimulus. The " physiological state," to 

 use Jennings' phrase, is altered, and a habit is es- 

 tablished. 



