ANIMALS AND THEIR ENVIRONMENTS. 



235 



indeed, the accidental occurrence of such a spon- 

 taneous transformation is far from probable. But 

 if this is not so, if the transit was gradual, then 

 how such transit of one eye a minute fraction 

 of the journey toward the other side of the head 

 could benefit the individual is, indeed, far from 

 clear. It seems even," concludes Mr. Mivart, 

 " that such an incipient transformation must 

 rather have been injurious." As far as these re- 

 marks regarding the rarity of sudden variations 

 are concerned, they are perfectly appropriate; 

 although it must, at the same time, be borne in 

 mind that occasionally startling modifications 

 have appeared in a species of animals in one gen- 

 eration, and without the slightest warning or in- 

 dication that a sudden alteration was to be pro- 

 duced. A well-known instance of this kind was 

 the sudden appearance of the ancon or otter 

 sheep of Massachusetts — a sheep possessing a 

 long body and short legs, 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 variations 

 in the flatfishes arose gradually, and were as 

 gradually intensified and transmitted as distinct 

 characters to their descendants. While, 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 ad- 

 vantage 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 favor the exist- 

 ence of the species. Unfavorable 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 va- 

 riation in the flatfishes was beneficial, and not 

 injurious, 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 standpoint, will an- 

 swer the demands laid upon him by opposing 

 tenets and theories. 



It may be observed, in the first place, that 

 the flatfishes are, to an appreciable and in a 



readily - understood sense, gainers from their 

 ground-inhabiting tendencies. Their bodies, as 

 already remarked, closely approach the color of 

 the sand and other surroundings, and they not 

 only find protection from their enemies in this 

 fashion, but i*eadily 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 ob- 

 servation, however, throws no light on the ques- 

 tion of the manner in which the modifications of 

 body which so perfectly adapt them for a ground- 

 life have been gained ; and, to attain the desired 

 information on this latter point, we must once 

 again study the early history of these fishes. 

 When young and possessing symmetrical bodies, 

 and when the eyes are placed in the natural situ- 

 ation, they may be observed to swim through the 

 water in a vertical position, like other fishes ; 

 their flattened surfaces appearing as their sides, 

 and the long dorsal and anal fins bordering the 

 upper and lower margins of the body respective- 

 ly, while the tail-fin is set vertically. Soon, how- 

 ever, it is observed that they retain their vertical 

 position in the water with difficulty, owing to the 

 great relative depth of their bodies. Like crank 

 ships, in fact, they have a tendency to become 

 overbalanced ; and there can be little doubt that 

 the small size of the pectoral and ventral fins, to- 

 gether with the absence of a " swimming-blad- 

 der," or " sound," materially aid in producing 

 this result. Thus unable to swim erect for any 

 length of time, the young flatfish comes to a 

 natural enough position of rest on its side. Malm's 

 observations now come to aid our comprehension 

 of the case in a very remarkable degree. This 

 observer tells us that the young fish, as it lies on 

 its side, twists the lower eye upward as if in the 

 effort to see above ; or, in plain language, tries to 

 look round the corner of its own head. So stren- 

 uous are these efforts of the young animal that 

 the eye is pressed with a great degree of force 

 against the upper part of the orbit or eye-cavity, 

 with the result, as Malm testifies, of contracting, 

 in a marked fashion, the forehead or space be- 

 tween the eyes. This observer, indeed, mentions 

 that he has witnessed a young flatfish elevate and 

 depress the lower eye through a distance corre- 

 sponding to an angle of seventy degrees. The ef- 

 fect of this frequent muscular exertion on the soft 

 cartilaginous and flexible tissues of the skull of 

 the young fish may readily be imagined. In time, 

 the temporary displacement of the tissues caused 

 by the movements of the lower eye comes to ex- 

 ercise a permanent influence in producing a de- 



