680 LECTURE XXIII. 



fied, the modification is almost as certainly transmitted by 

 sexual as by asexual reproduction. 



Without attempting to enter upon the large question of 

 the Origin of Species, we will briefly discuss one or two 

 points bearing on the subject which have been raised in the 

 course of this lecture. When varieties arise, it is assumed 

 that those which are most perfectly adapted to the prevailing 

 conditions persist by natural selection, whereas those which 

 are less perfectly adapted die out. By the gradual extinction 

 of these varieties, the intermediate forms between the per- 

 sistent varieties disappear; hence the persistent varieties 

 appear to be so distinct from each other that they rank as 

 species. Similarly, by the extinction of intermediate species, 

 species come to rank as genera. 



There can be no doubt that the multiplication of plant- 

 forms has taken place in this way, but the question has been 

 raised whether or not this is an adequate account of the evo- 

 lution of more and more highly organised forms ; for more 

 complete adaptation to the environment by no means in- 

 volves a higher organisation. For instance, we may account 

 for the various forms of the lower Algse as being the out- 

 come of variation and natural selection, but can we account 

 in this way for the evolution of the higher Algse, of the 

 Mosses, or of the Vascular Plants ? It may be urged that 

 variations in the direction of higher organisation may take 

 place, and may be gradually rendered permanent. But it is 

 questionable if the evolution of plants could ever, by this 

 means, have reached the point to which it has attained, 

 especially when we consider that much of it must have 

 taken place in forms reproducing themselves by the sexual 

 method, in which, therefore, in order for the variations to 

 have been made permanent and hereditary, the same varia- 

 tion must have occurred in many individuals simultaneously. 

 There seems to be some ground for believing that the evo- 

 lution of plants is the expression of something more than 

 fortuitous variation. As already mentioned (p. 657) Naegeli 

 suggests, and his suggestion is worthy of serious considera- 

 tion, that there is an inherent tendency to a higher organ- 



