SECniAE VAEIATIONS. 201 



spicuously in the different oceans and inland seas ; some water 

 is rich in oxygen or carbonic acid, in others these are wanting ; 

 in some cases we find large quantities of calcic carbonate, magnesio 

 sulphate, and other salts in the water, which is then termed 

 hard ; in others, as is usually the case with rain-water, these 

 tialts are almost entirely absent. 



All the differences here briefly enumerated, and many others 

 not specified, which affect those conditions of existence which 

 depend on air and water, must have more or less in- 

 fluence on the forms of animal life ; some species will abso- 

 lutely die out, others will remain wholly unaffected, while 

 others again will become modified in their habits of life as well 

 as in the structural relations of their organs (for instance, 

 Branchipus and Artemia). Now, if we assume that these 

 variations must be perfectly inappreciable to our individual 

 perceptions — secular variations, as they are termed — the modi- 

 fications which are caused by these secular variations in the 

 animal life of any given country must also be inappreciable by 

 man ; the apparent constancy of the conditions of life, so far as 

 they depend on air and water, will make the fauna appear 

 equally constant to our unaided vision. This hypothetical 

 constancy does actually exist, if we disregard the variations 

 occurring within the space of a day, a month, or a year ; the salt 

 constituents of the different oceans and inland seas remain 

 perfectly identical, as well as the moisture contained in the air 

 or in the composition of the atmosphere; we are in no way 

 cognisant of any perceptible variations in the conditions of 

 existence within the historical epoch of our globe. Hence we 

 may withoirt hesitation assume that any alteration caused by 

 such variations in the fauna of any locality or in the mode of 

 life of any animal and in the structure of its organs can never 

 have been perceptible to us. 



Nevertheless these two conditions of existence, air and 

 water, are precisely those which are most constantly at work 

 on animal types, and which are also the best qualified of any to 

 bring them into ever fluctuating conditions of existence, and 

 it is their capability for being moved of which nature avails 

 herself to effect the constant transfer of animals from one 

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