THE ARGUMENT FROM EMBRYOLOGY 79 



on the doctrine of Evolution that we have to consider 

 Embryology. 



There are two great questions to be considered : — 

 First : Does embryology afford evidence for or 

 \against the possibility of the descent of animals from 

 'unlike ancestors ? Secondly : If it gives evidence in 

 favour of such descent, does it afford us any clue in 

 regard to the actual line of descent in a given case, 

 and will it help, us ta~xeconstruct the pedigrees or 

 past histories of animals ? 



The answer to the first question is found in the 

 extraordinary changes which an animal may undergo 

 in its own person, during development, within the 

 space of a few days or weeks, thus showing the possi- 

 bility of such descent with modification ; for instance, 

 the changes which occur during the metamorphosis of 

 the butterfly, and the change of the water-breathing 

 tadpole into the air-breathing frog. This suggests 

 further that such enormous periods of time as are 

 usually dema nded to bring about sucTF~chariges may 

 not really be, necessary. A further reply to the 

 question is found in the fact that groups of animals, 

 the relations of which were previously unknown, 

 have had their true zoological positions determined 

 by the study of the changes undergone during their 

 development. 



Thus many animals, when adult, present little or 

 no resemblance to other members of the groups to 

 which they really belong. The Ascidiansarea well- 

 known example. So long as their adult condition 

 alone was known, zoologists were entirely in the 

 dark as to their real affinities, and by most writers 



