THE VEGETABLE AND ANIMAL KINGDOMS. 131 



they became gigantic tadpoles, and the reptile characters are 

 not developed. Here the progeny of a reptile literally be- 

 comes a fish, and transition of species is thoroughly realized, 

 although in retrogression. And this is an instance in which 

 the whole animal is concerned. Now surely no one will deny 

 that that which we see nature undo she is able to do, and 

 might be seen doing, if the proper occasion were to occur, or 

 were the requisite attendant conditions realized. 



So much with regard to grade. Let us now consider the 

 principle of modifidbility that part of the hypothesis to which 

 we are to look for an account of the external variations and 

 adaptation of animals. 



Here we are directly opposed by the prevalent doctrine 

 among naturalists, that species is intransible, and has so 

 continued during all the time that scientific observation 

 has existed. There is a certain volubility, they admit, in 

 organisms, throughout successive generations, and for this 

 variability external conditions may account ; but such varia- 

 tions show a disposition to give way, when the original con- 

 ditions are resumed, or when the changed individual is 

 mingled in alliance with the original stock. There is there- 

 fore a fixed and immutable character which we call species, 

 and which can only be traced to an origin differing as an event 

 from the procedure of nature in our own time. I trust to be 

 able to show that this doctrine is in such a condition with 

 regard to facts, and has of late been obliged by facts to make 

 so many shifts in its assumptions, that it is not entitled to the 

 respect usually assigned to it. 



The doctrine theoretically attaches the term species to 

 every organism which manifests the same peculiarities 

 throughout a series of generations. Practically, without 

 waiting to watch successive generations, or where, from the 

 extinction of the organism, this may be impossible, natura- 

 lists give the appellation to every organism which presents a 

 plurality of individuals similarly marked. Very small pecu- 

 liarities suffice. A particular spot on the wing of a butterfly 

 constitutes it a distinct species. The Golden Plover of Aus- 

 tralia was so reckoned, and got the name of Xanthocheilus, 



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