22 EVOLUTION OF CELLULAR STRUCTURES. 



is the network of descent which represents the concrete, significant 

 fact, and it is this which can be resolved, if necessar^^, into its compo- 

 nent lines, pol} g'ons, or nodes, to furnish units for the calculation of 

 (juantitative effects of inheritance, as in Galton's Law of Ancestral 

 Resemblances and Filial Regression, under conditions of normal sym- 

 basic descent, or in Mendel's Laws of Disjunction, in hybrids of 

 abnormally inbred varieties. 



The recognition of the double character of the cells of the higher 

 plants and animals permits many other phenomena of inheritance to be 

 understood, though it seems to take us farther than ever from the hope 

 of a merely mechanical explanation of the nature of heredity itself. 

 If conjugation were concluded immediately, the well-known phenom- 

 ena of sterile hybrids would be impossible, the sterility which puts an 

 end to their existence being due, as now known, to the failure to 

 perform synapsis or chromatin fusion. On the other hand, it may be 

 that crosses between narrowly inbred varieties sometimes have the 

 power of passing by the period of synapsis without a true fusion of 

 the parental chromatin, perhaps in a manner corresponding to that in 

 which Thalictrum produces seeds parthenogenetically, by avoiding 

 chromosome reduction. The germ-cells might have a preponderance 

 of chromatin from one parent or the other, or might even be quite 

 unmixed, as claimed for Mendelian hybrids. It is obvious, however, 

 that to explain Mendelism in this manner is to admit the essential 

 abnormality of the phenomenon. 



SUMMARY. 



It has been held self-evident that there can be nothing in evolution 

 except heredity and environment, and it was a simple deduction from 

 such an aphorism that differences must all be due to environment, since 

 " heredity would, if nothing interfered, keep the descendants perfectly 

 true to the physical characters of their progenitors.'"' Such heredit}^, 

 however, is a pure figment of the scientific imagination; it is a 

 hypothesis which lends us no aid in understanding the facts of 

 organic succession. A stereotyped heredity could make nothing new; 

 the interbreeding of diverse individuals and the prepotency' of new 

 variations are the constructive factors, not heredity and enviromnent. 



Symbasis is the method, interbreeding the means, and sexuality the 

 mechanism whereby organic evolution has been accomplished; these 

 are the concrete and efficient causes of the vital motion of species. 

 The association of organisms into species of similar individuals is not 

 brought about by a predetermining hereditary mechanism, but })y 

 S3^m basic interbreeding. The highest organization has not l)een 

 attained in "asexual generations," but in structures completely and 

 essentially sexual, built wholly of conjugating cells. Therehas been 



