114 THE GENEALOGY OF ANIMALS iv 



malian species whatever in wliicli the male nor 

 mally suckles the young. Thus, there can be 

 little doubt that the mammary gland was as 

 apparently useless in the remotest male mam 

 malian ancestor of man as in living men, and yet 

 it has not disappeared. Is it then still profitable 

 to the male organism to retain it ? Possibly ; but 

 in that case its dysteleological value is gone. 1 



II. Professor Haeckel looks upon the causes 

 which have led to the present diversity of living 

 nature as twofold. Living matter, he tells us, is 

 urged by two impulses : a centripetal, which tends 

 to preserve and transmit the specific form, and 

 which he identifies with heredity ; and a centri 

 fugal, which results from the tendency of external 

 conditions to modify the organism and effect its 

 adaptation to themselves. The internal impulse 

 is conservative, and tends to the preservation of 

 specific, or individual, form ; the external impulse 

 is metamorphic, and tends to the modification of 

 specific, or individual, form. 



In developing his views upon this subject, 

 Professor Haeckel introduces qualifications which 

 disarm some of the criticisms I should have been 

 disposed to offer ; but I think that his method of 

 stating the case has the inconvenience of tending 

 to leave out of sight the important fact which is 

 a cardinal point in the Darwinian hypothesis 



1 [The recent discovery of the important part played by the 

 Thyroid gland should be a warning to all speculators about 

 useless organs. 1893.] 



