472. THE HISTORY OF BIOLOGY 



its existence extremely hard to explain, but puts it most closely in connexion 

 with the age of the qualities in question. Also "latent," or, as they are called 

 nowadays, "recessive" qualities, he made the subjects of observation and 

 speculation. Particular care was devoted by him to the problem of hybridi- 

 zation; he is all the time procuring from his experiments proofs of the over- 

 lapping of varieties and species. Further, he investigated the cross and 

 self-fertilization of plants, which he was to deal with more fully in a subse- 

 quent special work. His speculations on fertilization and hybridization should 

 not be judged by modern standards; he knew as little as his contemporaries of 

 the true course of fertilization and so easily became deeply involved in specu- 

 lations as to the consequences of the effect upon the egg of adequate or inade- 

 quate quantities of sperm. He then discusses his favourite theory of the laws 

 of variation, which he now considerably expands, with an increasing ten- 

 dency towards Lamarckism, external circumstances — climate, food, and 

 even the use and non-use of organs — being definitely stated as influencing 

 the forms of variation. Even hybridization and atavism are cited as causes of 

 variation, besides which the phenomena of correlation are more closely 

 analysed in connexion with variability. 



Pangenesis 

 The anxiety to find a universally applicable explanation of the phenomena 

 of heredity and variation led Darwin to think out what he called a "pro- 

 visional hypothesis of pangenesis." In this theory he gives to the cytology 

 of the time, with which he otherwise had had nothing to do, a new and 

 curious interpretation. He believes that every cell, every tissue- or organ-unit 

 in the body, produces and gives off minute "atoms," which he calls gem- 

 mules, and that these latter, scattered throughout the body by the currents 

 of blood and other juices, conjoin as required, and then re-create those 

 "units" from which they are derived. The sexual products thus contain 

 ' 'gemmules" from all parts of the body, and these are combined in the embryo, 

 and it is for that reason that all the latter's parts resemble those of the father 

 or mother, according to whose gemmules have constructed the part of the 

 body in question. Unused gemmules may be transmitted to the next genera- 

 tion, with the result that some individuals resemble their father's or mother's 

 parents. In the same way the bud of a plant is formed by the gemmules of 

 those parts that are evolved out of it, and the regeneration of the severed 

 foot of a salamander takes place through the extremity gemmules accumu- 

 lating at the mutilated end; if, as sometimes occurs, a malformation takes 

 place, then the wrong gemmules have come into operation. This theory has 

 been shattered by modern research in the sphere of heredity and need not 

 therefore be discussed any further in this place; Darwin himself, it is true, 

 considered it to be only provisional, but he holds that it explains the prob- 

 lems at issue better than any other theory and should therefore be allowed 



