28 PROFESSOR A. M. MARSHALL. 



is primitive for vertebrates : i.e., whether it existed prior to the develop- 

 ment of gills. This point could be practically settled if we could decide 

 which of the two frogs, R. esculenta and R. temporaries,, has most 

 correctly preserved its ancestral history in this respect. 



About this there can be little doubt. The development of the 

 vessels in the newts, a less modified group than the frogs, agrees with 

 that of R. esculenta, and interesting confirmation is afforded by a single 

 aberrant specimen of R. temporaria, in which Mr. Bles and myself 

 found the vessels developing after the type of R. esculenta, i.e., in which 

 a complete aortic arch was present before the gills were formed. 



We are, therefore, justified in concluding that as regards the develop- 

 ment of the branchial bloodvessels, R. esculenta has retained a primitive 

 ancestral character which is lost in R. temporaria, and it is interesting 

 to note that were our knowledge of the development of amphibians 

 confined to the common frog, the most likely form to be studied, we 

 should, in all probability, have been led to wrong conclusions concern- 

 ing the ancestral condition of the bloodvessels in a point of considerable 

 importance. 



A matter which at present is attracting much attention is the 

 question of degeneration. 



Natural selection, though consistent with and capable of leading to 

 steady upward progress and improvement, by no means involves such 

 progress as a necessary consequence. All it says is that those animals 

 will, in each generation, have the best chance of survival which are 

 most in harmony with their environment, and such animals will not 

 necessarily be those which are ideally the best or most perfect. 



If you go into a shop to purchase an umbrella the one you select is 

 by no means necessarily that which most nearly approaches ideal 

 perfection, but the one which best hits off the mean between your idea 

 of what an umbrella should be and the amount of money you are 

 prepared to give for it : the one, in fact, that is on the whole best 

 suited to the circumstances of the case or the environment for the time 

 being. It might well happen that you had a violent antipathy to a 

 crooked handle, or else were determined to have a catch of a particular 

 kind to secure the ribs, and this might lead to the selection, i.e., the 

 survival, of an article that in other and even in more important 

 respects was manifestly inferior to the average. 



