THE FAMILY OFUNIONIDuE. l'l 



these species so merging, and, in some of their characters, so fading away into eadi 

 other, that we scarcely know how indeed, in some instances it is impossible to 

 make the separation with precision. "Natura non facit saltnm." A writer in the 

 Edin'bury'h Review, No. 212, p. 494, says some German ;m<l I-Yeneh botanists "content 

 themselves with calling species any collection of individuals \\liidi resemble c.idi 

 other more than they do any other set of individuals, without any limitation to tin- 

 degree of resemblance or difference which shall determine whether they belong to 

 the same or distinct species." 



"But the more rational definition adopted in substance by all the great masters 

 of the science of the present day is that of 'a collection of individuals \\hieh, by 

 their resemblance to each other, or by other circumstances, we are induced to believe 

 are all descended or may Jiave descended from one individual or pair of individuals.' 

 The various modifications of this definition enumerated by De Candolle consist 

 chiefly in the more or less detailed enumeration of the other circumstances, such a- 

 hereditary constancy of character, facility of intermixture of breeds," &c. 



In the vegetable kingdom, the same obstructions to a system are encountered. 

 The observations of Linclley 1 arc so just and philosophic, that I cannot refrain from 

 quoting them here : 



"Species are created by Nature herself, and remain always the same, in whatever 

 manner they may be combined: they form the basis of all classification, and are the 

 only part of it which can be considered absolute. For although, in a natural system, 

 all other combinations whether genera, tribes, orders, or by whatever name they 

 may be known comprehend species agreeing much more with eaeh other than with 

 anything else, and having a positive general resemblance in the majority of their 

 features, yet no fixed limits can be assigned to any of them; on the contrary, they 

 pass, by means of various intermediate species, into the other genera, tribes, order*. 

 &c., to which they are most nearly allied. For this reason, viz., that no fixed limit- 

 can be assigned to orders, genera, &c., we find the ideas about them fluctuating with 

 the degree of our knowledge; which is the true cause of those changes in the limits 

 of genera, &c., which persons unacquainted with the subject are apt to consider 

 arbitrary, but which, in skilful hands, are dependent upon a progressive advance in 

 the knowledge of science." 



Limueus defines species thus: "Species tot sunt quot diversas formas ab initio 

 produxit Infinitum Ens; qua3 forma? sccundum generationis indita- legc-. prodtU 

 plures, at sibi semper similes." Phil. Hot., 99, l.~>7. 



1 Sec Introd. lo M<>lun>/. \>. '->01. 



