12 THE WORLD OF LIFE CHAP. 



new species appears to be going on before our eyes. The 

 reader may therefore rest assured that there is no mystery 

 in the word species, but that he may take it as meaning the 

 same as kind, in regard to animals and plants in a state of 

 nature, and that he will have no difficulty in following the 

 various discussions and expositions in which this term is 

 necessarily so prominent. The reason why species is the 

 better term is because kind is used in two distinct senses 

 that of species when we speak of kinds of deer, of squirrels, 

 or of thrushes, but also that of a genus or a family when we 

 speak of the deer, squirrel, or thrush kind, as meaning the 

 whole group of these animals. If we used the word tribe 

 instead of kind in this latter sense, all ambiguity would be 

 avoided. 



Few persons who have not studied some branch of 

 natural history have any idea of the vast extent, the infinite 

 variety, the omnipresence and the intermingling of the 

 varied species of animals and plants, and still less of their 

 wonderful co-adaptation and interdependence. It is these 

 very characteristics that are least dwelt upon in books on 

 natural history, and they are largely overlooked even in 

 works on evolution. Yet they form the very basis of the 

 phenomena to be explained, and furnish examples of 

 development through survival of the fittest, on a larger 

 scale and often of easier comprehension than the special 

 cases most frequently adduced. It is this ground-work of 

 the whole subject that we will now proceed to consider. 



The Distribution of Local and World Species 



The first important group of facts which we have to 

 consider is that which relates to the number of existing 

 species of the two great divisions of life, plants and animals, 

 and their mode of distribution over the earth's surface. 



Every one who begins to study and collect any group 

 of animals or plants is at once struck by the fact that 

 certain fields, or woods, or hills are inhabited by species 

 which he can find nowhere else ; and further, that, whereas 

 some kinds are very common and are to be found almost 

 everywhere, others are scarce and only occur in small 

 numbers even in the places where alone they are usually 



