Method of Origin of New Species and Structures 415 



tuberculosis, but the weak lungs which render tuberculosis pos- 

 sible. Taken as a whole, therefore, the evidence we possess upon 

 the subject does not tend to support the Lamarckian theorj^ 



The contrast between the theories of Darwin and of Lamarck 

 is given the sharpest definition by the work of Weismann, an 

 eminent German zoologist still living. Darwin himself, while 

 convinced that natural selection was the leading factor in effect- 

 ing evolution, was inclined, especially in later life, to admit some 

 transmission of acquired characters; and indeed he actually in- 

 vented a special theory (called pangenesis, a flow of tiny solid 

 particles from all parts of the body to the germ cells), to explain 

 a possible mode of its operation; but his follower Weismann stood 

 for Natural Selection, pure and simple, as against the rival theory, 

 and even invented an ingenious conception to explain the natural- 

 ness of the operation of the one and the impossibility of the other. 

 In -brief, he held that there are two kinds of protoplasm in each 

 animal, — one reproductive, the germ plasm, confined to the eggs 

 and the sperm cells, and the other the body plasm, making up all 

 the rest of the organism. Now the fertilized egg-cell, from which 

 the new individual grows, is obviously germ plasm. As it grows 

 and develops, a part of the resultant cells keep on being germ 

 plasm, which, however, remains latent until the animal is adult, 

 while the remainder of the cells develop into body plasm, which 

 grows immensely and comes ultimately to surround, protect and 

 nourish the embedded germ plasm. Then when the time for 

 reproduction has arrived, it is always the latent germ plasm, 

 never the body plasm, which builds the new egg-cells and sperm 

 cells, whose union starts another indi\adual in the same way as 

 before. Thus germ plasm produces body plasm, but body plasm 

 never produces germ plasm. Hence the germ plasm is potentially 

 immortal, keeping on as one continuous line of tissue from genera- 

 tion to generation, while the body plasm is mortal, made anew in 

 each generation and perishing utterly therewith. The matter can 

 be expressed also in this manner, — that the germ plasm forms a 



