THE YOUNG NATUEALIST. 



29 



retrogression of the latter towards the reptilian type. 



All flying birds possess a keeled breast bone as did also the fossil flying 

 reptiles. 



All existing running birds have not a keeled breast bone, therefore such 

 birds are a retrogression to the reptilian type. 



Sloughing of reptiles and moulting of birds are identical processes. 



Some fossil and existing reptiles have air prolongations into the bones, as 

 in all birds. 



Geological evidence so far as it goes supports the evolutionary theory, in- 

 asmuch as reptiles precede birds. 



The deficiencies of the Geological record (where they exist) may be explain- 

 ed by the fact, that only a small portion of the earth has been examined by 

 scientific geologists — for as Darwin says the earth's crust may be likened to 

 a great museum of which only a few rooms have been examined. 



Fossil birds have reptilian tails. Hesporornis which succeed Arch- 

 iEOPTERYX, has cocygean bone, as have all subsequent birds. 



From the foregoing considerations, it will be clear that even in existing 

 forms the limits of zoological groups are difficult to define, and the study of 

 extinct forms only renders the definition more difficult. As a matter of fact, 

 a complete knowledge of all the living forms that have inhabited our earth, 

 would shew one complete series, ranging from a simple cell of protoplasm to 

 the highest verterbrate. It may perhaps never be possible to demonstrate 

 this absolutely, for intermediate forms would necessarily not last long, and 

 our knowledge of the geological record is after all very limited ; many species 

 having been developed, flourished for a time, then disappeared and left no 

 sign, except in the existence of their descendants. But after a careful study 

 of any single order, much less of the whole animal kingdom, no one can 

 doubt the truth of the evolutionary hypothesis. 



SLUGS AND THEIR VARIETIES. 



By Dr. J. W. WILLIAMS, M.A. 

 Editor of " The Naturalists' Monthly." 

 Continued from page 9. 



Limax Tbnelltjs. — This slug is a " North Britisher." A slender little 

 creature it is, never more than an inch in length. We will characterize his 

 specific differences from the other slugs in the same manner as we did in our 

 last paper, 



