202 THE INFLUENCE OF INANIMATE SURROUNDINGS. 



place to another. The passive migrations of animals are 

 effected entirely by the winds or currents, and their voluntary 

 movements are limited or favoured by them. Finally, the 

 strength and direction of these currents in the air and water 

 provide nature with so many instruments for influencing 

 different animals in their individual life and growth ; among 

 those so influenced, above all, are those which we may call 

 sedentary, in which a marked effect on the mode and vigour 

 of their growth can be traced to the moving medium. 



We will begin our enquiry into these important problems 

 by investigating the influences of water in motion. But it 

 must once more be pointed out that the influences discussed in 

 our former chapters must always be inseparable from those 

 proper to the currents, so that the total effect of the water arises 

 from the combination of several influences, not one of which 

 need ever act in the same direction as any other ; on the con- 

 trary, they frequently neutralise each other. If, for instance, 

 the larvae swimming in a sea were, without exception, drifted 

 by the same current simultaneously into an estuary, they would 

 apparently be thus enabled to take possession of the new 

 territory ; nevertheless all those forms would die out which 

 were not at once able to endure the reduced saltness of the 

 water in the estuary. A stream of warmer water, as, for instance, 

 the Mozambique Channel flowing past the east coast of Africa, 

 will have a tendency to convey animals of warm latitudes into 

 the colder seas ; but only a few species — those we have designated 

 as Eurythermal — will be able to establish themselves in them 

 easily. It is true that the difference in the conditions of life 

 under migration by means of sea-currents is not always so 

 conspicuous as in these extreme cases ; but all creatures are 

 exposed to changes of less intensity, if, in the larva or in the 

 fully grown stage, they are borne from one place to another. 



If we now leave out of the question those influences of the 

 constituents and temperature of the water which are inseparable 

 from its currents, and direct our attention solely to these, the 

 mechanical factor of their momentum is what we have to 

 consider as exclusively important. The direction, the rapidity, 

 and the strength of the cvirrent unite to affect the animals 



