406 BOTANICAL BIOLOGY. 



minute microscopic study of the reproductive processes. Ui)on these, 

 indeed, the correct classification of the Vascular Cryptogams wholly 

 depen<ls, and generally, as we descend in the scale, external morphology 

 becomes more and more insecure as a guide, and a thorough knowledge 

 of the niiuute structure and life history of each organism becomes in- 

 dispensal)le to anything like a correct determination of its taxouomic 

 position. The marvellous theory of the true nature of lichens would 

 never have been ascertained by the ordinary methods of exauiiuation 

 which were held to be sufficient b}' lichenologists. 



The final form of every natural classification — for I have no doubt 

 that the general principles I have laid down are equally true in the 

 field of zoology — must be to approximate to the order of descent. For 

 the theory of descent became an irresistible induction as soon as the 

 idea of a natural classification had been firmly grasped. 



In regard to fiowering plants we owe, as I have said, the first step in 

 a natural classification to our own great naturalist, John Kay, who 

 divided them into Monocotyledons and Dicotyledons. The celebrated 

 classification of Linnaeus was avowedly purely artificial. It was a tem- 

 porary expedient, the provisional character of which no one realized 

 more thoroughly than himself. He in fact himself gave us one of the 

 earliest outlines of a truly natural system. Such a system is based on 

 affinity, and we know of no other explanation of affinity than that 

 which is implied in the word, — namely, common parentage. No one 

 finds any difficulty in admitting that where a number of individual 

 organisms closelj" resemble one another, they must have been derived 

 from the same stock. I allow that in cases where external form is 

 widely different, the conclusion to one who is not a naturalist is by no 

 means so obvious. But iu such cases it rests on the profound and con- 

 stant resemblance of internal points of structure. Anyone who studies 

 the matter with a perfectly open mind finds it impossible to draw a 

 line. If genetic relationship or heredity is admitted to be the explana- 

 tion of affinity in the most obvious case, the stages are imperceptible 

 when the evidence is fairly examined, by which the same conclusion is 

 seen to be inevitable, even in cases where at the first glance it seems 

 least likely. 



This leads me to touch on the great theory which we owe to Mr. Dar- 

 win. That theory, I need hardly say, was not merely a theory of 

 descent. This had suggested itself to naturalists in the way I have 

 indicated, — long before. What Mr. Darwin did was to show how by 

 perfectly natural causes the separation of living organisms into races 

 which at once resemble and yet differ from one another so profoundly, 

 came about. Heredity explains the resemblance ; Mr. Darwin's great 

 discovery was that variation worked upon by natural selection ex- 

 plained the difference. That explanation seems to me to gather strength 

 every day, and to continually reveal itself as a more and more efficient 

 solvent of the problems which present themselves to the student of 



