SCIENCE. 



[N. S. Vol. XI. No. 284. 



facility which the individual acquires in 

 resisting temptation makes it any easier for 

 the OjEfspring to do the same, whether the 

 effects of education are cumulative in suc- 

 cessive generations. To put the matter as 

 Francis Galton has put it, is nature stronger 

 than nurture, or nurture than nature ? 



We have then two kinds of variation to 

 consider : (1) genetic variation, (2) ac- 

 quired modification. It is the former of 

 these — -name] 3% genetic variation — with 

 which I wish primarily to deal. Let us 

 examine more fully the mode of its occur- 

 rence. 



GENETIC VARIATION. 



Organized beings present, as you are 

 aware, two main kinds of reproduction, 

 the sexual and the asexual. These two 

 kinds of reproduction present certain differ- 

 ences, of which the most important, and 

 the only one which concerns us now, is the 

 fact that genetic variation is essentially as- 

 sociated with sexual reproduction, and is 

 rarely, if ever, found in asexual reproduc- 

 tion. In other words, whereas the offspring 

 resulting from asexual reproduction as a 

 rule exactly resemble the parent, thej"^ are 

 always different from the parents in sexual 

 reproduction. I am aware that I am tread- 

 ing on disputed ground. You will observe 

 that I do not make the assertion that asex- 

 ually produced offspring always exactly re- 

 semble the parent, and never present ge- 

 netic variations. To say that would be 

 going too far in the present state of our 

 knowledge. Therefore I have put the 

 matter less strongly, and merely assert 

 that whereas asexual reproduction is on 

 the whole characterized by identity between 

 the offspring and the parent, sexual repro- 

 duction is always characterized by differ- 

 ences more or less marked between the two ; 

 and I reserve the question as to whether 

 genetic variations are ever found in asexual 

 reproduction for later consideration. 



This modified form of the statement will, 



I think, be admitted on all hands, but be- 

 fore going on I will illustrate my meaning 

 by reference to actual examples. 



Asexual reproduction is a phenomenon 

 comparatively rare in the animal kingdom, 

 and when it does occur it is exceedingly 

 difficult to investigate from this particular 

 point of view. In the vegetable kingdom, 

 on the other hand, it is quite common. All, 

 or almost all, plants possess this power, and 

 in a very great many of them the result of 

 its exercise can be fully followed out, and 

 contrasted with that of sexual reproduction. 

 Let us follow it out -in the potato-plant. 

 The potato can and does normally propa- 

 gate itself asexually by means of its under- 

 ground tubers. As you will know, if you 

 take one of these and plant it, it gives rise 

 to a plant exactly resembling the parent. 

 If the tuber (seed as it is sometimes erro- 

 neously called) be that of the Magnum Bo- 

 num, it gives rise to a plant with foliage, 

 flowers and tubers of the Magnum Bonum 

 varietj^ ; if it be the Snowdrop, the foliage, 

 flowers, habit and tubers are totally differ- 

 ent from the Magnum Bonum, and are 

 easily identified as Snowdrops. In this 

 way a favorable variety of potato can be 

 reproduced to almost any extent with all 

 its peculiarities of earliness or lateness, 

 pastiness or mealiness, power of resisting 

 disease and so forth. By asexual repro- 

 duction the exact fac-simile of the parent 

 may always be obtained, provided the con- 

 ditions remain the same. 



Now let us turn to the results of sexual 

 reproduction — the seeds, i. e.,the real seeds, 

 which as you know are produced in the 

 flowers, are the means by which sexual re- 

 production is effected. They are produced 

 in great quantity by most plants, and when 

 placed in the ground under the proper con- 

 ditions they germinate and produce plants. 

 But these plants do not resemble the parent. 

 Try the seed of the Magnum Bonum potato 

 and raise plants from it. Do you think 



