Chap. VI. OF ORGANIC BEINGS. 175 



here alluded to, ■\vliicli j)c*rliiips may all have resulted from dis- 

 use, indicate the natural stops by whicli birds have acquired 

 their perfect jK)wer of ilifj^ht ; but they serve, at least, to show 

 what diversified means of transition are possible. 



Seeing that a few members of such water-breathing classes 

 as tlie Crustacea and Mollusca are adapted to live on the land; 

 and seeing that we have flying birds and mammals, lining in- 

 sects of the most divei-sified types, and formerly had Hying rej)- 

 tiles, it is conceivable that liying-flsh, Mhich now glide far 

 through the air, slightly rising and turning by the aid of their 

 fluttering llns, might liave been modified into perfectly-winged 

 animals. K this had Ix^en effected, who would have ever im- 

 agined that in an early ti-ansitional state they had been the in- 

 habitants of the open ocean, and had used their incipientorgans 

 of flight exclusively, as far as wc know, to escape being de- 

 voured by other fish ? 



AVhen m'C see any structure liighlj' perfected for an}- par- 

 ticular habit, as the wings of a bird for llight, we should bear 

 in mind that animals displaying early transitional grades of the 

 structure will seldom exist at the present day, lor they will 

 have been supplanted by their successors, which Avere gradu- 

 ally rendered more perfect through natural selection. Further- 

 more, we may conclude that transitional states between struct- 

 ures fitted for very difl'erent habits of life Avilkrarely have been 

 developed at an early period in great numbers and under many 

 subordinate forms. Thus, to return to our imaginary illustra- 

 tion of the llying-fish, it does not seem probable that fishes 

 capable of true flight would have been develoi^ed under many 

 subordinate forms, for taking prey of many kinds in many 

 ways, on the land and in the water, imtil their organs of flight 

 had come to a high stage of perfection, so as to have given 

 them a decided advantage over other animals in the battle for 

 life. Hence the chance of discovering species with transition- 

 al grades of structure in a fossil condition will always be less, 

 from their having existed in lesser numbers, than in the case 

 of species with fully-developed structures. 



I will now give two or three ihstances of diversified and of 

 changed habits in the individuals of the same species. In 

 either case it Avould be easy for natural selection to adapt the 

 structure of the animal to its changed habits, or exclusively to 

 one of its several habits. It is, however, dilhcult to decide, and 

 immaterial for us, whether habits generally change first and 

 structure afterward ; or Avhether slight modifications of struo 



