THE DISTRIBUTION OF ANIMALS AND PLANTS 415 



was colder than at present. Another curious case is that of the 

 marsh-tit of Europe. This little bird is found throughout south- 

 western Europe. It reappears in China, but is not known 

 anywhere between. In Siberia and northern Europe there 

 is, however, a species or distinct race which connects these 

 isolated patches. In this case, if the Siberian species is truly 

 distinct, we have a remarkable case of isolation and of the 

 permanence of identical characters for a long time ; for in that 

 case this bird must be a survivor of the Pliocene or Miocene 

 time. On the other hand, if, as is perhaps more likely, the 

 marsh-tit is only a local variety of the Siberian species, we have 

 an illustration of the local recurrence of this form when the 

 conditions are favourable, even though separated by a great 

 space and long time. 



The study of fossils gives us the true meaning of such facts, 

 and causes us to cease to wonder at any case of local repetition 

 of species, however widely separated. The " big trees " of 

 California constitute a remarkable example. There are at 

 present two very distinct species of these trees, both found 

 only in limited areas of the western part of North America. 

 Fossil trees of the same genus (Sequoia) occur as far back as 

 the Cretaceous age ; but in this age ten or more species are 

 known. Nor are they confined to America, but occur in 

 various parts of the Eurasian continent as well. Two of the 

 Lower Cretaceous species are so near to the two modern ones 

 that even an unbeliever in evolution may suppose them to be 

 possible ancestors ; the remaining eight are distinct, but some 

 of them intermediate in their characters. In the Tertiary 

 period, intervening between the Cretaceous and the modern, 

 fourteen species of Sequoia are believed to have been recog- 

 nised, and they appear to have existed abundantly all over the. 

 northern hemisphere. Thus we know that these remarkable 

 Californian giants are the last remnant of a once widely distri- 

 buted genus, originating, as far as known, in the Cretaceous age. 



