THE WEAK SIDES OF NATURAL SELECTION. 73 



are beginning to perceive that this is too difficult and complicated 

 a question to be decided either in this generation, or, in fact, in 

 any future generation without a far greater use of the methods of 

 observation and experiment than has hitherto been made. As he 

 remarks, the theory does not touch some of the simplest of 

 phenomena in the world around us. Granted for the moment 

 that in natural selection we have an explanation of the origin of 

 a species, I cannot see that that lands us very much on our way ; 

 the practical knowledge of the cattle-breeder, the pigeon-fancier, 

 and the horticulturist, in pre-Darwinian times carried them nearly 

 as far, only that they did not formulate a theory of the universe on 

 those grounds. 



Some reference is made by Mr. Slater to the extent in which 

 indigenous plants have been often exterminated by introduced 

 species, but we must also remember the destruction, far more 

 extensive, as T think, of would-be intruders into the domains of 

 previously established species, and the modification of others. For 

 nearly two years I have been making observations on the dispersal 

 of water-plants and marsh-plants, and for a long time I imagined 

 that the problem to be solved might be briefly thus stated : — ■ 

 " Given the distribution and capacity for dispersal of a plant, to 

 explain its distribution," but I gradually came to see that another 

 postulate was required. Take, for instance, the case of our 

 common marsh-plant, Bidens cernua. It is rarely that one finds in 

 the same plant to the same degree equal capacities for dispersal 

 by the different agencies of the currents, birds, etc., etc. The 

 achenes can float for months in sea-water and yet germinate ; 

 they float all the winter through in our rivers, such as the Lea, 

 and must be transported in great numbers annually to the sea, 

 when they commence their ocean voyage. The reflexed prickles 

 of the achenes eminently fit them also for transportal in birds' 

 plumage, for which they are as well adapted as the fruits of Galium 

 aparine, and I cannot doubt but that birds such as duck, teal, &c, are 

 very important agents in the dispersal of this plant. Yet with all 

 these means of dispersal, this plant, though diffused widely in the 

 temperate and northern regions of Europe, Asia, and America, is 

 not to be found in the tropics. I do not doubt for a moment but 

 that the achenes of this plant have been transported to almost 

 every corner of the globe a thousand times over, and vet the species 

 is not to be found in the tropics. And why not ? Either the con- 

 ditions there are antagonistic, or else it has sported in its new 

 home into varieties that owe their permanence to their surround- 

 ings, and so we call them " species." The genus, as we learn 

 from Bentham's and Hooker's handbook, is not very numerous in 

 species and is diffused over the whole globe, occurring even in the 

 Arctic Circle. 



We thus perceive that the absence of a plant in a particular 

 region may be by no means due to its inability to get there. We 



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