42 



have changes wrought in its body. Xow since its body is at the same time 

 the reprodueUce element, whatever modifications have arisen in the body 

 would be inherited by the two portions into which it would divide. " If," 

 says Weismann, " a Protozoon, by constantly struggling against the me- 

 chanical inrtuence of currents in water, were to gain a somewhat denser 

 and more resistant protoplasm, or were to acquire the power of adhering 

 more strongly than the other individuals of his species, the peculiarity 

 in question would be directly continued on into its two descendants, for 

 the latter are at first nothing more than the two halves of the former." 

 By the time, therefore, that some of the Protozoa, through more and more 

 intimate association into colonies, by differentiations of the cells for the 

 performance of different functions, and the production of germ-cells as 

 distinguished from the body-cells, became modified into the primitive 

 Metazoa, those individual differences had arisen which, constantly multi- 

 plied ever since by sexual mixture, have furnished the materials on 

 which Natural Selection has worked to produce all the living animal 

 forms that now exist. 



It must be understood that, as regards the reproductive elements of the 

 higher animals, AVeismann contends for the continuity of the germ-plasm, 

 not for that of the germ-cells. Embryology proves that the latter cannot 

 be maintained. As Weismann says, " continuity of the germ-cells does 

 not now take place, except in very rare instances." In certain insects 

 there are, at the very beginning of development, a few cells separated 

 from the others and afterwards received into the body of the embryo, in 

 order later to develop into eggs. In some crustaceans, the germ-cells be- 

 come distinct when about thirty cells have been produced. In verte- 

 brates they do not usually become distinct from those composing the body 

 until the embryo has been completely formed. Among the Hydroids, re- 

 production occurs largely by budding. The buds may develop into inde- 

 pendent bodies, jelly fishes, which swimming away and attaining a large 

 size, give origin to the germ cells. These do not make their appearance 

 until after hundreds and thousands of cell -generations have been passed 

 through. They arise oi'iginally from certain cells of the ectoderm, but 

 make long migrations to the places whei'e they finally undergo develop- 

 ment into perfect eggs. Among plants, a fertilized ovule gives origin to 

 an embryo. This may develop into a large tree, which finally will, at the 

 tii3s of branches a hundred feet away, produce new ovules. Through 

 millions of cells the germ-plasm must have made its way to reach those 



