ANIMALS AND THEIR ENVIRONMENTS. 309 



which was produced as the offspring of an ordinary ewe and 

 ram. This sheep in its turn became the progenitor of a 

 whole race of ancons ; and many other examples of sudden 

 variations from the type of a species might be illustrated in 

 both animal and plant worlds. But apart from the fact that 

 alterations of structure, as great as those seen in the flat-fishes, 

 have been suddenly developed in animals, Mr. Mivart is 

 correct enough in laying stress on the fact that, to satisfy Mr. 

 Darwin's ideas, it must be proved to be likely that the varia- 

 tions in the flat-fishes arose gradually, and were as gradually 

 intensified and transmitted as distinct characters to their 

 descendants. Whilst, if Mr. Darwin's theory is tenable, it 

 must also be shown that the propagation of such deviations 

 from the ordinary structure of the fishes, was an advantage 

 to the animal concerned. In this last thought, indeed, lies 

 the essence and strength of Darwinism. Nature selects 

 such variations for transmission to posterity as will favour 

 the existence of the species. Unfavourable variations will, 

 in the " struggle for existence," tend to die out. Hence Mr. 

 Mivart most appropriately calls upon the supporters of the 

 theory of evolution by " natural selection " to show cause 

 that the variation in the flat-fishes was beneficial and not in- 

 jurious to the individuals exhibiting it. Such are the issues 

 of the question before us. Let us try to discover how the 

 evolutionist, viewing the question from the Darwinian stand- 

 point, will answer the demands laid upon him by opposing 

 tenets and theories. 



It may be observed in the first place that the flat-fishes 

 are, to an appreciable and in a readily understood sense, 

 gainers from their ground-inhabiting tendencies. Their 

 bodies, as already remarked, closely approach the colour of 

 the sand and other surroundings, and they not only find 

 protection from their enemies in this fashion, but readily 

 obtain food from the sand on which they rest. As far as 

 the advantages gained from their habits are concerned, the 

 case seems clear enough if regarded in this light. This 

 observation, however, throws no light on the question of the 



