XXI.] EMBRYOLOGY. 275 



offered of the manner in which stages of development 

 have been dropped ont, and the process of development 

 thereby shortened. But this is no explanation of the Objection. 

 Origin of Species. The difficulty is not to knoAV how 

 metamorphosis and metagenesis have been suppressed, but 

 how they began. You tell us how the land salamander^ 

 has ceased to begin life as a tadpole : but how did the 

 first salamander make the transformation from a water- 

 breathing tadpole into an air-breather ? You tell us how 

 the Aphis has ceased to appear as a worm ; but how was 

 the first worm transformed into a hexapod insect ? You 

 tell us how the fresh-water Crustacea have ceased to leave 

 the egg as zoeas ; but what determined the first transfor- 

 mation of a zoea iato a crayfish ? You tell us how the 

 Lizzia form of Medusa has come to produce its like with- 

 out alternating with a hydra-like form ; but how did the 

 first hydra-like form begin to throw off Meduste ? Until 

 these questions, and such as these, can be answered, the 

 mystery is unsolved." 



I admit that what I have said leaves altogether unsolved Eeply. 

 the mystery of the origin of organic forms. I intend, in a 

 future chapter, to show how far I think that mystery is 

 capable of solution. But I cannot admit that we know 

 nothing in any case where we are unable to ascend to 

 a knowledge of causes. A true and correctly generalized 

 view of facts not only is valuable in itself, but is often the 

 means by which it becomes possible to ascend to the 

 knowledge of causes. The discovery of Kepler's laws is 

 justly regarded as one of the most important contributions 

 ever made to science, though Kepler died in ignorance of 

 the reasons of those laws; but Kepler's discoveries pre- 

 pared the way for N"ewton's discovery of the law of gravi- 

 tation, which has explained the reason, not only of Kepler's 

 laws, but of the apparent exceptions to them. 



But before going on to the subject of the causes of these 

 transformations, I shall endeavour to show how the deve- 

 lopment theory of the origin of species agrees with the 

 facts of classification. 



■^ Salamandra atra, or land-newt. 

 T 2 



