134 THE 'ORIGIN OF SPECIES.' [1860 



swering. Never think of this, for I look at every letter of 

 yours as an honor and pleasure, which is a pretty deal more 

 than I can say of some of the letters which I receive. I have 

 now one of 13 closely written folio pages to answer on spe- 

 cies ! . . . . 



I have a very decided opinion that all mammals must 

 have descended from a single parent. Reflect on the multi- 

 tude of details, very many of them of extremely little impor- 

 tance to their habits (as the number of bones of the head, &c., 

 covering of hair, identical embryological development, &c. 

 &c.). Now this large amount of similarity I must look at as 

 certainly due to inheritance from a common stock. I am 

 aware that some cases occur in which a similar or nearly 

 similar organ has been acquired by independent acts of nat- 

 ural selection. But in most of such cases of these apparent- 

 ly so closely similar organs, some important homological dif- 

 ference may be detected. Please read p. 193, beginning, 

 " The electric organs," and trust me that the sentence, " In 

 all these cases of two very distinct species," &c. &c., was not 

 put in rashly, for I went carefully into every case. Apply 

 this argument to the whole frame, internal and external, of 

 mammifers, and you will see why I think so strongly that all 

 have descended from one progenitor. I have just re-read your 

 letter, and I am not perfectly sure that I understand your point. 



I enclose two diagrams showing the sort of manner I con- 

 jecture that mammals have been developed. I thought a little 

 on this when writing page 429, beginning, " Mr. Waterhouse." 

 (Please read the paragraph.) I have not knowledge enough 

 to choose between these two diagrams. If the brain of Mar- 

 supials in embryo closely resembles that of Placentals, I 

 should strongly prefer No. 2, and this agrees with the antiq- 

 uity of Microlestes. As a general rule I should prefer No. i 

 diagram ; whether or not Marsupials have gone on being 

 developed, or rising in rank, from a very early period would 

 depend on circumstances too complex for even a conjecture. 

 Lingula has not risen since the Silurian epoch, whereas other 

 molluscs may have risen. 



