114 THE GENEALOGY OF ANIMALS IV 
malian species whatever in which 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.’ 
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 conteipeie 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 en 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.] 
