16 EVOLUTION OF CELLULAR STRUCTURES. 



stagnation, can well be relegated to the liinl)o of h^'potheses which 

 have not proved useful. Heredity is not a mechanism or a force; it 

 is merely another name for the property of organic continuity or suc- 

 cession. There is no more heredity in an organism at one stage of its 

 life history than at another. 



Sexual and other diversities inside specific lines are not useless 

 morphological complexities or mere failures in the execution of a 

 fundamental plan of complete uniformity. Diversity, interbreeding, 

 and evolution are physiological factors of the highest importance in 

 maintaining vital efficiency. 



Morphologically speaking, sexuality is a specialization of the inter- 

 nal diversity of the species, and among plants, at least, it has been 

 attained independently in a large num))er of unrelated natural groups. 

 There are grades of sexual difierentiation just as there are of organic 

 structures. Moss plants and fern prothalli may be sexually ditieren-, 

 tiated and the diti'erentiation may occur farther back in the spore 

 itself, or even in the sporophyte or double-celled phase, as in the 

 flowering plants and the higher animals. Thus in the same species 

 there may be two sexualities, one in the simphvcelled stage and 

 another in the double, and these may have no homology or causal con- 

 nection, except as they both serve the same purpose of promoting 

 more efficient symbasis. Indeed, the sexuality of the highest types of 

 organization is not merely double, but threefold; the individual has 

 sex, as a whole; the double cells of which the body is composed are a 

 part of a sexual process, and the simple cells which it produces for the 

 initiation of a new generation are sexually difi'erentiated. 



TWO TYPES OF DOUBLE-CELLED STRUCTURES. 



That organisms are everywhere associated in species is not because 

 of some undiscovered principle or mechanism of heredity; it is simply 

 because the interweaving of the lines of individual descent is being 

 maintained, without which the specific association would be dissolved 

 into indefinite radial divergence and degeneration, as among the varie- 

 ties of bananas and other plants long propagated from cuttings. Many 

 explanations have been conjectured for the supposed absence of sexual 

 reproduction among the higher groups of fungi. From the standpoint 

 of a symbasic evolution, however, it becomes evident that the exist- 

 ence of true, coherent species among these fungi is a sufficient evidence 

 of interbreeding, and hence of sexuality. There is in many groups a 

 deficiency of specialized sexual organs, but these are rendered unneces 

 sary by abundant opportunities for direct conjugation among the 

 mycelial filaments. 



That the cells of the more complex reproductive tissues of the higher 

 fungi are known to have two nuclei, while in the younger mycelium 



