128 SECRETS OF EARTH AND SEA 



excessive and useless outgrowth. (This astonishing 

 creature is shown in our Frontispiece.) Such exuberant 

 products may be ascribed to an unrestrained " momentum " 

 of growth which once set going by fortuitous variation has 

 been tolerated but not favoured by natural selection. 

 Or (as supposed by some) their excessive development 

 may be due to the persistence of some nutritional condi- 

 tion which at first resulted in a moderate growth of the 

 fin-like crests in question as a serviceable structure, but 

 has persisted and increased long after the fin or crest has 

 attained a sufficient size — simply because its increase 

 though of no life-saving value — yet was not harmful and 

 so did not bring its owner under the guillotine of natural 

 selection. Such disproportionate exuberance of growth 

 due to innate variability, tolerated but not specially 

 favoured by natural selection, will account for many 

 strange and grotesque forms of living things. From time 

 to time in the long process of change, such exuberances 

 may suddenly become of service and be, so to speak, 

 taken in hand by natural selection, or they may become 

 dangerous and lead to the extermination of the stock in 

 which they have been previously tolerated. 



Before my reader turns — as I hope he or she will do — 

 to some handbook of zoology in which the genealogical 

 tree or classification of the species of animals and of 

 plants is treated at length, I will endeavour to give some 

 estimate of the immense numbers of " species " which 

 exist. As to mere individuals, it is impossible to form 

 any estimate, but when we reckon up the teaming popula- 

 tion of a meadow or forest in England, the hundreds of 

 thousands of plants, including the smallest mosses and 

 grasses, as well as the larger flowers, shrubs, and trees, 

 the still greater number of insects, spiders, snails, and 

 larger animals and birds, feeding on and hiding among 

 them, and when we remember that in the ever-warm tropical 



