54 THE GERM-PLASM 



If in considering a theory of heredity we had only to deal with 

 an explanation of the transmission of an unalterable structure 

 from the parent to the offspring from generation to generation, 

 there would be theoretically no objection to the assumption of 

 such a structure of the germ-plasm. We have, however, to deal 

 with the transmission of parts which are variable, and this 

 necessitates the assumption that just as many independent and 

 variable parts exist in the germ-plasm as are present in the fully 

 formed organism. It is impossible that a portion of the body 

 should exhibit an independent variation capable of transmission 

 unless it were represented in the germ-plasm by a special par- 

 ticle, a variation in which is followed by one in the part under 

 consideration. If this were represented, together with other 

 parts of the body, by one particle of the germ-plasm, a change 

 in the latter would be followed by a variation in all the parts of 

 the body determined by it. The independently and hereditarily 

 variable parts of the body therefore serve as an exact measure for 

 determining the nnniber of ulti))iate particles of whicJi the ger?n- 

 piasm is composed: the latter must contain at least as great a 

 fiumber as would be arrived at by such a computation. 



An example may make it clear that the independently variable 

 parts are not identical with those which are merely hereditary. 



It is well known that butterflies pass through a metamor- 

 phosis in the course of development, the stages of which are 

 independently variable from the germ onwards : that is to say, 

 a variation in the caterpillar is not necessarily followed 

 by one in the butterfly, and vice versa. The caterpillars 

 of a species may be dimorphic, some being green, and 

 others brown, but both of these forms nevertheless give 

 rise to butterflies with a similar coloration. If, therefore, the 

 phyletic modifications depend on changes in the minute 

 structure of the germ-plasm, there must be at least two inde- 

 pendently variable units in the germ-plasm of such a butterfly ; 

 for if there were only one, the butterfly as well as the caterpillar 

 would be affected by a variation in it. But a comparison of 

 nearly related species shows us that the individual parts of the 

 caterpillar or butterfly must also be variable from the germ 

 onwards : the limbs, for instance, of two species may be very 

 similar, while their wings are different, and even the separate 

 parts of the wings may vary independently of one another. We 

 must therefore assume that the germ-plasm contains a large 



