Evolution Going On 203 



New Devices 



It is impossible, we must admit, to fix dates, except in a few 

 cases, relatively recent ; but there is a smack of modernity in some 

 striking devices which we can observe in operation to-day. Thus 

 no one will dispute the statement that spiders are thoroughly 

 terrestrial animals breathing dry air, but we have the fact of the 

 water-spider conquering the under-water world. There are a few 

 spiders about the sea-shore, and a few that can survive douching 

 with freshwater, but the particular case of the true water-spider, 

 Argyroneta natans, stands by itself because the creature, as 

 regards the female at least, has conquered the sub-aquatic en- 

 vironment. A flattish web is woven, somehow, underneath the 

 water, and pegged down by threads of silk. Along a special 

 vertical line the mother spider ascends to the surface and descends 

 again, having entangled air in the hairs of her body. She brushes 

 off this air underneath her web, wjiich is thereby buoyed up into 

 a sort of dome. She does this over and over again, never getting 

 wet all the time, until the domed web has become like a diving-bell, 

 full of dry air. In this eloquent anticipation of man's rational 

 device, this creature far from being endowed with reason lays 

 her eggs and looks after her young. The general significance of 

 the facts is that when competition is keen, a new area of exploita- 

 tion is a promised land. Thus spiders have spread over all the 

 earth except the polar areas. But here is a spider with some 

 spirit of adventure, which has endeavoured, instead of trekking, 

 tc find a new corner near at home. It has tackled a problem 

 surely difficult for a terrestrial animal, the problem of living in 

 great part under water, and it has solved it in a manner at once 

 effective and beautiful. 



In Conclusion 



We have given but a few representative illustrations of a 

 great theme. When we consider the changefulness of living 

 creatures, the transformations of cultivated plants and domesti- 



