INTESTINAL TRACT OF MAMMALS. 535 



is limited and defined. Whatsoever influences come to work upon it, however the 

 selection or the direct moulding of the environment may act upon the stock, the 

 possibilities are limited and defined by the past history. The changes must run along- 

 certain grooves. "Whatever forces operated on the diverging members of the Ungulate 

 stock, for instance, had, so to speak, to reckon with the definite Ungulate modification 

 that had taken place in the pattern of the intestinal tract. The changes and 

 elaborations that could take place in the intestinal tract of the Ungulates were so far 

 definitely limited ; the direction of the movement was limited and could lead only to 

 further specialisation of the limited material, or, if that were impossible, to extinction. 

 And the further along each radius a set of organisms moved, so the inevitable unlike- 

 ness from organisms on other radii must have increased. The existence of s-uch a 

 principle of the formation of definite grooves along which the specialisation of organisms 

 must necessarily move has been recognised by many writers ; but amongst these not a 

 few have endeavoured to read into the phenomena some mysterious, ultra-physical 

 directive force, or, on the other hand, to seek out " selection-value" for each anatomical 

 disposition. It is at least simpler first to exhaust the obvious interpretations, to be 

 certain that there is a mystery, before turning to a mysterious explanation. 



List of References. 



i. Bcddard, F. E. — Mammalia. Cambridge Natural History. 1932. 



2. Birmingham, A.- — Some Points in the Anatomy of the Digestive System. Journ. of Anat. 



xxxv. pp. 33-36. 



3. Burmeister, H. — Beitrage zur naheren Keimtniss der Gattung Tarsius. Berlin. 1816. 



4. Cuvier, G. — Lecons dAnatomie comparee. 2nd edition, 1835. 



5. Flower, Sir W. H.- — Lectures on the Comparative Anatomy of the Organs of Digestion of 



the Mammalia. Medical Times and Gazette, 1872. 



6. Garrod, A. H. — On the Brain and other Parts of the Hippopotamus. Trans. Zool. Soc. xi. 



pp. 11-17. 



7. Garrod, A. H. — On the Ccecum coll of the Capybara [Hydrochoerus capybara). Proc. Zool. 



Soe. 1876, pp. 19-22. 



8. George, M. — Monographic anatomique dcs Mammiferes du Genre Daman. Ann. Sci. Nat., 



Zoologic, 17 aim. ser. vi. vol. i. p. 252. 



9. Home, Sir E. — Anatomy of the Dugong. Phil. Trans. 1820, p. 3.15. 

 io. Hunter, W. — Essays and Observations, vol. ii. 



11. Ki.aatscii, II. — Zur Morphologie der Mesenterialbildungen am Darmkanal der Wirbelthiere. 



Morph. Jahrb. 1892, pp. 385-450, 609-716. 



12. Lische, W. — Ueber die Saugethiergattun^ Galeopilhecus. Kong. SvenskaVet.-Ak. Handl. xxi. 



1 886. 



