MOUKRX THEORIES OF HEREDITV. 3/3 



reproduction only certain cells take part, which are generally 

 known as genn-cclls; from these germ-cells, sometimes singly, 

 but usually onl}' after two cells derived from different individuals 

 have fused together, the embryo is formed. The embryo even- 

 tually displays to a greater or less degree the characters of the 

 parent or parents, and the question arises, how are the characters 

 of the different parts of the body handed on, when only the 

 germ-cells take part in reproduction? Darwin and his con- 

 temporaries and followers were of opinion that the germ-cells 

 were produced by the body, and that particles from every part 

 of the body entered into their composition. This is called the 

 doctrine of " pangenesis." 



W'eismann developed a quite contrary view, namely, that the 

 germ-cells are the important part of the organism, that the rest 

 of the body is only as it were an offshoot from the germ-cells, 

 and that changes in the characters of the body have no influence 

 whatever on the germ-cells. Weismann believed that in the 

 embryo the cells giving rise to germ-cells were, from the first, 

 distinct from those which give rise to the rest of the body, and 

 that the germ-cells, their ancestors and descendants, remain dis- 

 tinct throughout the life-history of the organism. Hence the 

 phrase " continuity of the germ plasm," which expresses the 

 central idea of Weismann's theory. Xo one doubts that, 

 ordinarily, the germ plasm is continuous (in the sense in which 

 the ])hrase is used) from one generation to another, but discus- 

 i<ion has centred in the question whether the, germ-cells may be 

 modified or influenced by changes taking place in the body, 

 induced by external conditions. This question resolves itself 

 into another, which can be to some extent tested experimentally, 

 namely: Are "acquired characters" {i.e., those acquired by an 

 individual organism as the direct result of the action of external 

 conditions) inherited? There is not the least doubt that when 

 this questioli is submitted to strict experiment the answer must 

 be that, so far as can be seen in a limited number of generatio)i^„ 

 acquired characters are not inherited. The reservation may, 

 however, be a very important one, since, as we know, evolution 

 proceeds very slowly, and in nature the production of a per- 

 ceptible and permanent change of characters may well take a 

 much longer period of time than that assigned to any experiment 

 or series of experiments such as those designed to test this 

 question. To make the case clearer, the type of experiment 

 which would throw light on the inheritance of acquired characters 

 may be outlined. It is a matter of common knowledge that the 

 same species of ])lant often has quite different leaf characters 

 when grown in a dry or in a wet locality. Let us take a numb.er 

 of seeds from such a plant, and sow half in a wet, half in a dry 

 legality. Allow the plants so grown to set seed, and take seed 

 samples from each type of plant. Now choose a locality of an 

 intermediate character, and sow two samples of seeds side by 

 -^ide under ])recisely the same conditions. Then, if acquired 



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