INSTINCT 221 



hairs which inclose air, and this prevents it from 

 becoming wet. 



The cell having been constructed, the spider pro- 

 ceeds to fill it with air. The hind legs are covered 

 with hair, and they are of such shape that they can 

 secure a large bubble of air which it carries under the 

 cell and releases. The air rises to the top of the cell, 

 expelling the water. By repeating the process the 

 cell is filled with air. In the upper part of the cell 

 the eggs are placed and surrounded by a cocoon. 

 About one hundred young are hatched and reared in a 

 single cell. 



In this case we see several things which must have 

 been produced simultaneously if they were evolved. 

 First, the peculiar hind legs of the animal which ena- 

 ble it to carry a large bubble of air would have been 

 of no use to it for this purpose without its habit of 

 living under water, and consequently this structure 

 would not have been evolved in the absence of the 

 habit. But it could not live under the water without 

 a cell nor without the instinct to fill the cell with air. 

 Each of these two instincts, together with the peculiar 

 structure which enables it to carry air with its legs, 

 would have been useless without the other, and conse- 

 quently, if they were evolved, they were produced 

 simultaneously. They must all have been produced 

 before the spider could live habitually under water. 



The instinct and the power to construct a perfect 

 cell through which water would not pass and the in- 

 stinct to fill it with air could not have been gradually 

 acquired by natural selection, for they must all be per- 

 fect before the spider can live under the water. An 

 imperfect cell, or an imperfect instinct failing to 

 properly fill it with air, or inability to perform the 

 work owing to defective structure would be fatal in 

 attempting to make the change from a terrestrial to 



