REPRODUCTION AND HEREDITY 311 



individual of a species is only able to reproduce new individuals 

 of the same species. The theory of heterogeneous generation 

 was soon abandoned as a gross error wherever it was promul- 

 gated. Thus it is a universally accepted principle of biology 

 that ' like produces only like,' or better, * species always produces 

 its species.' Among all unicellular organisms homoeokinesis 

 of their cell-organisms is the only method of division that occurs 

 and can occur. On it rests the constancy of the species. If 

 it were possible in any unicellular organism to divide the heredi- 

 tary substance (idioplasm) into unequal component parts and 

 transmit them unequally to the daughter-cells we should have 

 a case of heterogeneous generation the origin of two new 

 species from one species. But, as all observations teach us, the 

 characters of the species are even in the unicellular organisms 

 transmitted by division with such minute care that unicellular 

 fungi, algae and infusorians, even in the millionth generation, are 

 exactly like their ancestors. The process of division, as such, 

 appears therefore, never as a means of generating new species, 

 not even in the unicellular organisms. 



' For these reasons it seems to me inadmissible to regard 

 cell-division in the ovum as a means of obtaining diametrically 

 opposed objects, producing now like, now unlike. Here, 

 too, each cell-division can in its very nature be only a 

 homoeokinetic one, therefore all the cells that develop from the 

 ovum by reproduction must be carriers of the entire idioplasm, 

 and of equal species.' 



Weismann's theory is further met by almost insuperable 

 difficulties in the phenomena of regeneration and asexual repro- 

 duction by division and gemmation. As the germ-plasm in the 

 course of the development of the ovum is, according to him, 

 divided into its determinants and, as it were, used up, it can 

 naturally only be capable of producing the single body-parts and 

 organs once. But, as we have seen, numerous lower animals 

 may be divided into several parts, and yet each part may develop 

 once more into a complete organism ; other animals are at least 

 able to repair lost extremities; the single cell of a begonia leaf is 

 able to reproduce the entire plant ; and finally the ability, under 

 favourable conditions of reproducing the species must, in the 

 opinion of most botanists, be ascribed to every plant-cell : the 



