34 THE AGE OF MAMMALS 



Again, the archaic, or Cretaceous placcntals of North America and 

 Europe, although alike marked by extremely low organization in certain 

 characters, in other characters more or less closely imitate the radiations oi 

 higher groups and give us bear-like, cat-like, dog-like, and hysena-like 

 forms. 



Connecting this principle with the laws of adaptive radiation, conti- 

 nental and local, we find that the radiations in different areas are more or 

 less analogous with each other; that is, we discover many analogous radii 

 or lines of adaptation, among other radii which are entirely dissimilar. 

 When we come to compare the early evolution of the mammals in Africa, 

 for example, we shall find that adaptation pursued entirely different lines 

 from those pursued in Europe, Asia, and North America; so that when 

 the African mammals finally entered Europe, after having undergone a 

 long independent evolution of their own, they were entirely dissimilar and 

 foreign in appearance to any with which they competed in Europe. 



One of the most important advances of the past twenty years has been 

 the clear recognition of this law of analogy and of the pitfalls which it con- 

 stantly spread for the earlier students of mammals. It may be described 

 as the very "will o' the wisp" of evolution, always tending to lead the 

 student of descent astray. 



The Law of Irreversibility of Evolution 



A very frequent feature of divergent adaptation is the loss of parts as 

 explained on p. 15, or the very profound modification of parts, as in the 

 "tree phase," of the early life of the marsupials, in which two of the toes 

 become syndactylous, or closely applied to each other. These lost parts 

 are never reacquired, nor can such profound modifications of form and 

 proportion be overcome; a specialized organ can never again become 

 generalized, lost parts are irretrievable. It follows that while the condi- 

 tions of hfe may be recurrent or reversible, the conditions of adaptive 

 structure are not reversible. Hence the dictum of Dollo ^ that evolution, 

 while frequently reversible in conditions of environment and adaptation, 

 is irreversible in animal structure. Each part that is lost, like a tooth 

 or a digit, narrows do\vn the possibility of future plastic adaptation to 

 new conditions. Nature often resorts to other remedies to repair her 

 losses, namely, to substitution of parts, or to change of function. 



Thus extreme specialization accompanied by the great enlargement 

 of certain parts and the great reduction of other parts often places a 

 mammal in a cut de sac of structure, where it is incapable of further 

 modification to meet a new environment. This may become a cause of 

 extinction. 



» Dollo, Les Lois de rEvolution. Bull. Soc. Beige Geol, Paleont., HydroL, Vol. VII, 1893, 

 pp. 164-166. 



