ENVIRONMENT, PAST AND PRESENT 73 



the northern hemisphere as we know it now would 

 mean a return to glaciation. A deviation of but 

 2°C. above or below the present mean would induce 

 marked climatic change. The fossil record, even 

 though it be one of constant evolution as far as 

 certain groups of animals are concerned presents 

 us with an unbroken thread of life. Here we note 

 the disappearance of species that have survived for 

 hundreds of thousands of years, there the first ap- 

 pearance of others and everywhere the inconceiv- 

 ably slow transformation of one form into another. 

 But at the same time there are species existing 

 today that have successfully survived unchanged 

 for millions of years. The creation of one, the 

 extinction of another, the constant change that we 

 term evolution, these things reflect a locally chang- 

 ing environment. The effective changes, great or 

 small, may have been of a thousand different kinds 

 but in the survival of numerous forms, unchanged 

 and resembling exactly their progenitors of millions 

 of years ago, we find evidence that bears but one 

 interpretation — the essential conditions required by 

 living organisms have remained unaltered through 

 the ages. They are undoubtedly the same today as 

 they were at the dawn of life. The fundamental 

 requirements of the birds of the present epoch were 

 also those of the earliest Eocene species. We are 



