SUMMARY OF 'PHILOSOPHIE ZOOLOGIQUE: 287 



less regular according as the organ is of less import- 

 ance, and is more susceptible of modification by the 

 conditions which surround it. Organs of small import- 

 ance, and not essential to existence, are not always 

 either perfected or degraded at an equal rate, so that 

 in observing all the species of any class we find an 

 organ in one species in the highest degree of perfection, 

 while another organ, which in this same species is 

 impoverished or very imperfect, is highly developed in 

 another species of the same group." * 



The facts maintained in the preceding paragraph 

 are in great measure supported by Mr. Charles Darwin, 

 who, however, assigns their cause to natural selection. 



Mr. Darwin writes, " Ordinary specific characters are 

 more variable than generic ; " and again, a little lower 

 down, " The points in which all the species of a genus 

 resemble each other, and in which they differ from 

 allied genera, are called generic characters ; and these 

 characters may be attributed to inheritance from a com- 

 mon progenitor, for it can rarely happen that natural 

 selection will have modified several distinct species 

 fitted to more or less widely different habits, in exactly 

 the same manner ; and as these so called generic cha- 

 racters have been inherited from before the period 

 when the several species first branched off from their 

 common progenitor, and subsequently have not varied 

 or come to differ in any degree, or only in a slight 

 degree, it is not probable that they should vary at the 

 present day. On the other hand, the points in which 

 species differ from other species of the same genus are 

 * 'Phil. Zool.,' torn. i. p. 122. 



