io6 SECRETS OF EARTH AND SEA 



carries the plague bacillus from animals to man, and is 

 a cause of death ; other species, extremely like it in ap- 

 pearance, but distinguishable by a trained observer, do not 

 carry the plague bacillus, but if they swallow it, destroy it 

 by digestion. One species of gnat, the common grey gnat, 

 digests and destroys malaria germs when it sucks them up 

 with blood ; in an allied species, the spot-winged gnat or 

 Anopheles, the chemical juices of the gut allow the germ to 

 live in it and multiply, and so to be carried to men by the 

 gnat's bite. So with many other flies and parasites the 

 recognition of the dangerous species is of vital importance, 

 and that recognition often depends on minute features of 

 form and colour not at once obvious to an ordinary 

 observer. 



But this recognition of distinct species is, from the 

 point of view of the study of Nature, only a preliminary 

 to the question, " How did these species come about ? 

 How is it that there are so many species, some very like 

 one another, forming genera, and these genera grouped 

 into related families, these into larger groups, and so on, 

 like the branches of a family tree ? " The answer to these 

 questions given by Linnaeus was: "There are just so 

 many species as the Infinite Being created at the beginning 

 of things, and they have continued to propagate themselves 

 unchanged ever since." The answer which we give to-day 

 is that the appearance of a huge family tree which our 

 classification of animals takes is due to the simple fact 

 that it really is neither more nor less than a family tree 

 or pedigree — the " tree of life," of which the green leaves 

 and buds are the existing species. Further, we hold that 

 the existing species of a genus have " come into existence " 

 by natural birth from one ancestral species, its offspring 

 having slightly varied (we are all familiar with this 

 individual variation in our own species, in dogs, cats, trees, 

 and shrubs), and that the varieties have wandered apart 



