ORIGIN AND CHARACTERISTICS 13 



Hornbill, a Plantain-eater (Necrornis), an Adju- 

 tant (Leptoptilus), certain forms of Cuckoo 

 (Centropus), and Trogons (Trogonidae). By 

 most zoologists the presence of such species in 

 Europe is taken as an indication that the climate 

 at that time was much warmer than it is now, 

 or even of a tropical character. This may or 

 may not have been the case ; but to suggest that 

 these birds (now only found in warm latitudes) 

 were driven south by the cooler climate of the 

 succeeding Pliocene era, or the intense cold that 

 characterised that of the Glacial Epoch, is not, 

 in my opinion, a correct interpretation of the 

 facts. I maintain that one of the laws of dis- 

 persal is that species never retreat from adverse 

 conditions of life. All such species that during 

 later Tertiary time had spread north and east 

 from southern and even equatorial bases, and 

 become sedentary in the warm equable climate 

 of Euro-Asia must have been completely ex- 

 terminated by the advent of adverse conditions 

 of life, primarily due to a change of climate. 

 Let it then be clearly understood that these 

 avine types now^ extinct in Europe did not 

 originate there during the long-continued warm 

 climatic conditions of the later Tertiary Period, 

 that they did not at the approach of a cooler 



