286 KEY TO TERRESTRIAL HISTORY 



circulation and more rapid oceanic currents ; floating 

 forms would be swept along by more swiftly-moving 

 vehicles of transport, and would spread along the sea- 

 chore and pass from island to island with a velocity 

 which might perhaps be twice as great as at present. 



Those organisms which pass through a free swimming 

 larval state of existence would be affected as well as those 

 which float by means of a pneumatophore or in other 

 ways. The developmental history of the organisms prob- 

 ably differed in the past in a manner favouring a wider 

 distribution than they now enjoy, for it is a universal 

 tendency among such forms to shorten in the course of 

 successive generations the period of a free larval exist- 

 ence ; possibly in order that all the advantages of the 

 adult state may be acquired as early as possible. If this 

 be so, and there can be little doubt of it, then as we pro- 

 ceed backwards in time, we shall find that sessile organisms 

 will pass a longer and longer time in their probationary 

 locomotive stage ; 20 millions of years ago, perhaps as 

 many weeks as now days might have been expended in 

 a state of free larval existence. It might even occur to 

 some one to suggest that, in this case, the Graptolites 

 might have been dispersed over the whole world merely 

 as a consequence of a protracted existence in the larval 

 state. This, however, is not likely ; the equatorial 

 current of the Atlantic, by no means a slow stream as 

 ocean currents go, has a speed of from eight to eleven 

 miles a day ; supposing this increased to twenty miles, 

 as the result of changed conditions, then it would require 

 just a hundred days proceeding at this speed to cross 

 the Atlantic in a straight line in its narrowest part, that 

 is, between Sierra Leone and Brazil ; so that, unless we 

 invent a sufficient number of islands as stepping-stones 

 on the way, even comparatively long-lived larvae would not 

 be likely to make the passage across deep oceans as these 



