xi.] THE GENEALOGY OF ANIMALS. . 275 



in general do, that useless organs atrophy, such cases as 

 the existence of lateral rudiments of toes, in the foot of 

 a horse, place us in a dilemma. For, either these rudi- 

 ments are of no use to the animal, in which case, con- 

 sidering that the horse has existed in its present form 

 since the Pliocene epoch, they surely ought to have 

 disappeared ; or they are of some use to the animal, in 

 which case they are of no use as arguments against 

 Teleology. A similar, but still stronger, argument 

 may be based upon the existence of teats, and even 

 functional mammary glands, in male mammals. Nume- 

 rous cases of " Gynsecomasty," or functionally active 

 breasts in men, are on record, though there is no mam- 

 malian species whatever in which the male normally 

 suckles the young. Thus, there can be little doubt 

 that the mammary gland was as apparently useless in 

 the remotest male mammalian 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 \ Pos- 

 sibly ; 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 centripetal, which tends to preserve and 

 transmit the specific form, and which he identifies with 

 heredity; and a centrifugal, 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 



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