D.— ZOOLOGY 97 



as the wing, which unlocked the kingdom of the air to the birds ? 

 This is in one sense true ; but in another it is untrue. The birds, 

 although they did conquer a new section of the environment, in so 

 doing were as a matter of actual fact cut off from further progress. 

 Theirs was only a specialisation. The conquest of the land, however, 

 not only did not involve any such limitations, but made demands upon 

 the organism which could be and in some groups were met by further 

 changes of a definitely progressive nature. Temperature- regulation, for 

 instance, could never have arisen through natural selection except in an 

 environment with rapidly changing temperature. 



As revealed in the succession of steps that led to new dominant forms, 

 progress has taken diverse forms : at one stage, the combination of cells 

 to form a multicellular individual, at another the evolution of a head ; 

 later the development of lungs, still later of warm blood, and finally the 

 enhancement of intelligence by speech. But all have, though in curiously 

 different ways, increased the organism's capacities for control and for 

 independence ; and each has justified itself not only in immediate results 

 but in the later steps which it made possible. 



So much for the fact of progress. What of its mechanism ? It will 

 be clear that if natural selection can account for adaptation and for long- 

 range trends of specialisation, it can account for biological progress too ; 

 for progressive changes have obviously given their owners advantages. 

 Sometimes it needed a climatic revolution to give the progressive change 

 full play, as at the end of the Cretaceous with the mammal- reptile differ- 

 ential of advantage : but when it came, the advantage had very large 

 results — -wholesale extinction on the one hand, wholesale radiation of 

 new types on the other. It seems to be a general characteristic of evolution 

 that in each epoch a minority of stocks give rise to the majority in the next 

 phase, while, conversely, of the rest the majority become extinguished 

 or are reduced in numbers. 



There is no more need to postulate an elan vital or a guiding purpose 

 to account for evolutionary progress than to account for any other feature 

 of evolution. 



One point is of importance. Although we can quite correctly speak 

 of evolutionary progress as a biological fact, this progress is of a particular 

 and limited nature. It is an empirical fact that evolutionary progress 

 can only be measured by the upper level reached ; for the lower levels are 

 also retained. It is of course a fallacy to use this fact as an argument 

 against the existence of progress. To do so is on a par with saying that 

 the invention of the automobile does not represent an advance, because 

 horse-drawn vehicles remain more convenient for certain purposes, or 

 pack animals for certain localities. 



One somewhat curious fact emerges from a survey of evolutionary 

 progress. It could apparently have pursued no other course than that 

 which it has historically followed. 



Multicellular organisation, triploblastic development, a ccelom and a 

 blood system were clearly necessary to achieve a reasonable level of size 

 and organisation. Among the ccelomates, only the vertebrates were eligible, 

 for only they were able to achieve the combination of active efficiency, size, 

 and terrestrial existence needed as a basis for the later stages of progress. 

 The arthropods are not only hampered by their moulting, but their land 



