Canon Tristram on the Polar Origin of Life. 211 



find the group represented in every region and in every dis- 

 trict of every region of the worlds excepting in New Zealand. 

 In habits the species vary from the widest limits of migra- 

 tion to the most limited localization. It would seem as 

 though the progenitors had pursued every available route as 

 they retired from the Pole. But the most southern and 

 sedentary species have indisputably a northern origin. Take, 

 for instance^ the Blackbird. We find, to mention only a few 

 species out of many, in Western Europe, in Eastern Asia, in 

 the mountains of Ecuador, in the Samoa Islands, Turdiis 

 merula, T. mandarinus, T. serranus, and T. samoensis, all 

 Blackbirds, differing only in size and in some slight pecu- 

 liarities, such as the colour of the legs. These must have 

 had a common ancestry, whose progenitors, we may sap- 

 pose, had become differentiated from any other type of 

 Turdidce before they left the Polar continent. It is impos- 

 sible to conceive that T. &erranus and T. merula were derived 

 the one from the other, or that the two have ever been in 

 contact since their progenitors left their ancestral home. 

 But if there were a Blackbird generally spread over the 

 Arctic continent, and dispersed, partly down the East At- 

 lantic line, and partly down the east and west coasts of the 

 Pacific, the progenitors of T. serranus and its neotropical 

 congeners must have travelled down by the Rocky-Mountain 

 range, till, reaching the higher Andes, they found on the moun- 

 tain-slopes the changes they required, and substituted a vertical 

 Ifor a latitudinal migration, according to the season. Similarly, 

 the progenitors of T. merula spread over Europe and became 

 migratory to a very limited extent ; while a third party, 

 skirting Eastern Asia, followed the mountain-ranges from 

 China westward ; and became slightly differentiated, as they 

 settled in various districts, into the numerous species of 

 Blackbirds of Eastern Asia. Some of this party, more adven- 

 turous than their fellows, appear to have crossed into Formosa, 

 and there slightly dwindling in size, partially acquired that 

 white plumage so often characteristic of insular forms, and 

 became the White-headed Blackbird, T. albiceps, of Formosa. 

 From this adventurous race seem to have sprung the many 



