342 THE 'ORIGIN OF SPECIES.' [iPCo. 



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 conjec- 

 ture 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 anti- 

 quity 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. 



A, in the following diagrams, represents an unknown form, 

 probably intermediate between Mammals, Reptiles and Birds, 

 as intermediate as Lepidosiren now is between Fish and 

 Batrachians. This unknown form is probably more closely 

 related to Ornithorhynchus than to any other known form. 



I do not think that the multiple origin of dogs goes against 



the single origin of man All the races of man are so 



infinitely closer together than to any ape, that (as in the case 

 of descent of all mammals from one progenitor), I should look 

 at all races of men as having certainly descended from one 

 parent. I should look at it as probable that the races of men 

 were less numerous and less divergent formerly than now, 

 unless, indeed, some lower and more aberrant race even than 

 the Hottentot has become extinct. Supposing, as I do for 

 one believe, that our dogs have descended from two or three 



