XXXVI GENERAL INTRODUCTION. 



As the whole question of reversion involves a general 

 acquaintance with animal pedigrees, I may, before re- 

 ferring further to my horse embryos, say a few words 

 about what is usually known as the Recapitulation Theory. 

 In the making of a bi'eech-loading hammerless gun, a 

 piece of wood, is gradually fashioned into a stock, and 

 various kinds of metal are manipulated in diverse ways to 

 form the lock and barrels. From the first it is evident 

 what the workmen are about. But if a gun were con- 

 structed on the plan followed by nature in the making of 

 a mammal, the procedure would be very diiferent. The 

 gun-maker (who would require to be a miracle worker as 

 well) would go to work in some such roundabout way as 

 the following. Starting with a piece of clay, he would 

 set about transforming it into flint, but before completing 

 the process would convert it into something remotely 

 resembling a boomerang or a knob-kerry. By further 

 legerdemain the " throwing stick " would become a 

 <;ross-bow, and this, by a long and intricate series of 

 transformations, would assume, or all but assume, in turn 

 the characters of a flint gun, various kinds of muzzle- 

 loaders, and breech-loaders with hammers. Eventually it 

 would take the form of a hammerless ejector. 



In no Vertebrate is the development direct, however 

 rapid it may be. For example, the horse, an all but in- 

 visible speck of protoplasm to stai-t with, passes at the 

 outset through a series of remarkable changes (which so 

 far can only be guessed at), to emerge at the end of the 

 third week as a somewhat fish-like creature almost bent 

 double over a large active heart. The tail is bilobed, 

 like that of a mermaid (manatee), but neither limbs nor 

 jaws have yet appeared. A mammal at no stage can 

 make use of gills, yet the horse embryo has, for a time, 

 three pairs of gill pouches. A quarter of i\\\ inch long at 

 three weeks, the horse embryo is half as long again at 

 four weeks. It might, at this stage, pass for a human 

 embryo of a similar age. There are paddle-like limbs, 

 and a lizard-like tail many times longer than the limbs. 

 At five weeks the limbs are still paddle-like, but in each 



