ARTICLES 395 



which it has acquired during its own life through the influence of 

 outward circumstances, such as cHmate, nourishment, training, 

 etc." 



The opinion that acquired characters are not inheritable, 

 and have therefore played no part in evolution, was first put 

 forward by Weismann. 



When we turn to what he says on the subject in " Germ- 

 Plasm," we are amazed to read : " It is easier to explain the 

 transformation of species on Lamarck's principle, but this is no 

 reason for the retention of a theory which cannot be accepted on 

 theoretical grounds, unless no other explanation can be given for 

 the facts." Weismann does not deny that the modifying effects 

 of climate are in some cases transmissible by heredity ; but he 

 endeavours to explain this away by the supposition that climate 

 acts directly on the germ-cells ; that is to say, he commits 

 himself to the extraordinary position that climate can directly 

 affect the germ-cells buried deep within the tissues of the body, 

 but that changes on the bodily tissues in which these germ- 

 cells are immersed are totally without effect on them 1 



His theoretical grounds against admitting the transmissi- 

 bility of acquired characters are : 



1. The view that the bearer of heredity is the chromatin 

 in the nucleus of the cell — as witnessed by the fact that the 

 whole of influence of the father on the character of the offspring 

 is carried by the head of the spermatozoon, which is a condensed 

 nucleus consisting only of chromatin. 



2. The view that the primary germ-cells which eventually 

 give rise to the genital cells of an individual are set aside at an 

 extremely early period in the development of the egg : that the 

 other cells which constitute the " soma " (body) pursue an 

 independent development, and hence that their subsequent 

 changes cannot affect the nature of the germ-cells, the 

 characters of which are fixed from the outset. 



Now, of these theories, No. i has been confirmed by later 

 research ; but No. 2 is really ludicrous. Even supposing, 

 which is not the case, that definite "germ-tracks" had been 

 shown to exist in the developing eggs of all animals — that is 

 to say, that it has been shown that the lineage of the germ- cells 

 had always been traceable to undifferentiated cells amongst 

 the blastomeres which result from the cleavage of the egg — that 

 would not prove that their hereditary potentialities were un- 

 alterable by changes in the body fluids which bathe them. 



But definite germ-tracks have been shown to exist in very 

 few animals ; the most evident case is in the development of 

 Nematoda. In these worms, as the egg divides, the nuclei of 

 some of the blastomeres undergo changes which appear to be 

 degenerative, and the cells to which these nuclei belong give 



