GENERAL INTRODCCTION. XXXVU 



paddle the foundations of three digits (2, 3, and 4) have 

 appeared. A week later the fore-limbs are longer, have 

 undergone rotation inwards, and the three digits in each 

 are more distinct. The jaws have now appeared, and 

 there is no longer any indication of gill pouches. In 

 the six-weeks horse (though under three quarters of an 

 inch in length) we have a partial restoration of the 

 earlier three-toed ancestors of the Equidae, but ere the 

 seventh week is reached all external indication of the 

 outer (2 and 4) digits has vanished, and the single re- 

 maining digit (3) is beginning to assume the characteristic 

 equine form. By the end of the eighth week, though 

 the embryo is barely two and a half inches in length, we 

 have in many ways a miniature horse, the limbs extended 

 as if ready for action, and the tail only reaching the 

 hocks. If at six weeks the cross-bow stage has, as it 

 were, been reached, by the eighth we have got as far as 

 a flint gun. 



As it happens, these and other facts to be mentioned 

 presently have a bearing not only on the reversion theory, 

 but also on a subject attracting much attention at tha 

 present time, viz. the origin of the three great groups of 

 Mammals. During recent years this subject has been 

 often discussed, and we seem to be drifting further and 

 further from the view that the higher Mammals (Eutheria) 

 have descended from the Marsupials — the opossum and 

 kangaroo group, — while they in their turn originated 

 from the egg-la3dng Monotremes, the group nov/ repre- 

 sented by the duck-mole and echidna. 



It may be more than a mere coincidence that, for a few 

 days, the horse embryo forcibly reminds one of some of 

 the young Marsupials. In nearly all Marsupials nourish- 

 ment up to the time of birth can only be derived from 

 food — '' uterine milk " — filtering into the yolkless yolk- 

 sac. This necessitates premature birth. In the opossum,. 

 e. g., the j-oung are transferred to the pouch as soon as 

 they can hang on to the teats. In the higher Mammals 

 and in certain Marsupials a more permanent plan is 

 adopted for the nourishment of the embryo. The embryo- 



