EXPLAINING THE CONTRADICTIONS. 135 



of the mother. If we find all the representatives of 

 a large group agreeing in some common feature, we 

 may usually conclude that it is ancestral. Organs 

 of no use to the embryo, such as the gills of the 

 Guadaloupe frog ; organs appearing and then dis- 

 appearing ; organs arising and being subsequently 

 converted into something else, may all be unhesi- 

 tatingly set down as features inherited from the 

 past. But the most important factor in enabling us 

 to decide in any given case, is the directness or 

 indirectness of the development. When we find 

 that the young is built in a straightforward manner 

 directly from the egg, we may rest assured that the 

 embryology does not repeat very exactly the history 

 of the past ; but when the formation of the embryo 

 takes place, by means of a long series of round- 

 about stages, we may strongly presume that some 

 or all of them are ancestral stages. An absolutely 

 direct development is not known, for every animal 

 passes through some out-of-the-way stages. The 

 development of the chick is extremely direct, but 

 even here many ancestral features are retained. On 

 the other hand, no animal is known whose embry- 

 ology is such as not to lead us to believe it to be 

 abbreviated in some respects. But at the same time 

 many do have such a roundabout history as to in- 

 dicate that in most points they repeat ancestral 

 features. When, further, it is found that those 

 animals with a direct development almost universal- 

 ly have a large amount of food-yolk in the egg, and 

 that those with an indirect development agree in 

 having little food, and are consequently not able to 



