CHAP. xxi. ALTERNATE GENERATION. 421 



externally, no visible departure from the normal form. Thus, 

 in one region a species may possess peculiar medicinal quali 

 ties which it wants in another, or it may be hardier and 

 better able to resist cold. The average range in altitude, says 

 Hooker, of each species of flowering plant in the Himalayan 

 Mountains, whether in the tropical, temperate, or Alpine 

 region, is 4,000 feet, which is equivalent to twelve degrees 

 of isothermals of latitude. If an individual of any of these 

 species be taken from the upper limits of its range and 

 carried to England, it is found to be better able to stand our 

 climate than those from the lower or warmer stations. 

 When several of these internal or physiological modifications 

 are accompanied by variation in size, habits of growth, colour 

 of the flowers, and other external characters, and these are 

 found to be constant in successive generations, botanists may 

 well begin to differ in opinion as to whether they ought to 

 regard them as distinct species or not. 



Alternate Generation. 



Hitherto, no rival hypothesis has been proposed as a sub 

 stitute for the doctrine of transmutation ; for ( independent 

 creation,' as it is often termed, or the direct intervention of the 

 Supreme Cause, must simply be considered as an avowal that 

 we deem the question to lie beyond the domain of science. 



The discovery by Sefstrom of alternate generation enlarges 

 our views of the range of metamorphosis through which a 

 species may pass, so that some of its stages (as when a Sertu- 

 laria and a Medusa interchange) deviate so far from others as 

 to have been referred by able zoologists to distinct genera, or 

 even families. But in all these cases the organism, after 

 running through a certain cycle of change, returns to the 

 exact point from which it set out, and no new form or species 

 is thereby introduced into the world. The only secondary 



