304 TRAXSACTIOXS AND PKOCEEDINGS OF THE [Sess. lxiv. 



of the vegetative system following on its loss of structur.il 

 differentiation and functional insignificance. This " time 

 displacement " is best seen in the specialised heterosporous 

 forms. 



For example, in Sclaginclla the male prothallus and 

 autheridium are contemporaneously developed almost, so 

 much has the rate of development of the prothallus 

 been retarded in consequence of functional and structural 

 insignificance. Similarly, in the female the sub-diaphrag- 

 matic part of the prothallus shows such retardation in its 

 development that it only appears after formation of the 

 sexual organs. Similar " time displacements " appear in the 

 angiosperms. The female prothallus- — the antipodals, — 

 insignificant both in structure and function, has had its 

 development so slowed down that it appears contempor- 

 aneously with the egg apparatus, and if the endosperm 

 be of a prothallial nature, then the " time displacement " 

 is still more marked. 



Beginning with the lowest plant forms,, we find 

 a.sexuality prevailing ; as the upward progress is followed, 

 sexuality is gradually introduced and gametophytes appear; 

 next, an introduction of the asexual phase recurs, and 

 sporophytes result and alternation. At the upper end 

 of the line, the sexual phase seems to be disappearing 

 as the sexual plant becomes a vanishing quantity. 

 The prevailing trend in the evolution of the higher 

 plants would seem to be towards the production of a 

 series of asexual plants, i.e. a return to the primitive 

 condition, but with a difierence. While in the lowest 

 types asexuality exists with a generalised and simple 

 type of plant body and conduces to permanence, in the 

 highest forms so evolved and specialised — over-specialised, 

 indeed, so far as tlieir system of multiplication is con- 

 cerned — asexuality, co-existing witli a highly specialised 

 vegetative system, would lead to extinction, after the 

 pliylogenetic cycle suggested above was complete. 



The results of cultivation, whicli may in a way be 

 looked upon as an artificial procef.s ol" evolution, bear on 

 this point. The tendency to sterility in highly cultivated 

 plants is known. Cultivated forms tend to hai'k liack 

 to the original type, being in a kind of unstaljle equi- 



