158 Thomas Henry Huxley 



more iiearl}- the two animals are related, when Huxley 

 wrote, was founded on a much smaller number of facts 

 than now are known. Since i860 an enormous bulk of 

 embryological investigation has been published, and 

 the total result has been to confirm Huxley's position 

 in the fullest possible way. A certain number of ex- 

 ceptions have been found, but these exceptions are so 

 obviously special adaptations to special circumstances 

 that their existence only makes the general truth of the 

 proposition more clear. The most common kind of ex- 

 ception occurs when two closely related animals live 

 under very different conditions. For instance, many 

 marine animals have close allies that in comparatively 

 recent times have taken to live in fresh water. The 

 conditions of life in fresh water are very different, espe- 

 ciall}^ for delicate creatures susceptible to rapid changes 

 of temperature, or unable to withstand strong currents. 

 Thus most of the allies of the fresh- water crayfish, which 

 live in the sea, lay eggs from which there are soon 

 hatched minute, almost transparent larvae, exceedingly 

 unlike the adult. In the comparative!}^ equable tem- 

 perature of sea-water, and in the usual absence of strong 

 currents, these small larvae, as Huxley shewed later in 

 his volume on the Crayfish^ live a free life, obtaining 

 their own food, and by a series of slow transformations 

 gradually acquire the adult form. In fresh water, how- 

 ever, the delicate larvae would be unable to live, and 

 the mode of development is different. The series of 

 slow transformations is condensed, and takes place 

 almost entirely inside the egg-shell ; so that, when 

 hatching occurs, the young crayfish is exceedingly like 

 the adult. Apart from such special cases, it is true 

 that the study of development affords a clear test of 

 closeness of structural affinity. 



