526 Reviews — Age and Area. 



and bands established by Professor Garwood in the ShajD area are 

 recognizable throughout the district. The various beds are often 

 shifted laterally, cut out, or repeated by a complicated system of 

 dip, strike, and block faulting. Mr. Edmonds has promised to 

 discuss the earth movements in the Egremont area in a subsequent 

 paper ; as they are somewhat similar to those in the northern 

 limestone area, they need not be discussed here. 



REVIEWS. 



Age and Area : A Study in GEoaRAPHicAL Distribution and 

 Origin of Species. By J. C. Willis, with chapters by 

 Hugo de Vries, H. B. Guppy, E. M. Reid, and James Small. 

 8vo, X + 260 pp. Cambridge University Press, 1922. Price 

 145. net. 



"D Y " Age and Area " Dr. Wilb's means that the area occupied by 

 ^-^ a given species is proportionate to its age. Area is calculated 

 .by taking the greatest distance from the assumed or inferred centre 

 of dispersal as the radius of a circle, that circle being regarded as the 

 area even though the distribution of the species within it be sporadic. 

 Age is the time that has elapsed from the entry of a species into the 

 district under consideration. If the district be an island (to take 

 a simple case) the species may have entered from a neighbouring 

 district where it previously existed, or it may have originated as a 

 new species within the island itself ; the centre of dispersal then is, 

 in the former instance, the point of entry, in the latter instance the 

 point of origin, and age is dated from entry or origin respectively. 

 The general statement is to be understood as referring to averages 

 of not less than ten species, and those species must be allied. Thus 

 interpreted, the statement is believed to be similarly applicable to 

 genera and to families, and to be equally true whether the district 

 under examination be an island, a continent, a geographical region, 

 or the whole world. Though based originally on the distribution of 

 land-plants, the principle can. Dr. Willis claims, be extended to 

 animals. The animals mentioned are mostly those of land or fresh 

 water, but there seems no reason why marine faunas and floras should 

 not be subject to the same rule. 



It would be easy to reel off a long list of exceptions to the 

 statement, even if attention were restricted to species now living. 

 But remembering the careful qualifications by Dr. Willis, and noting 

 the evidence brought forward by him and by others, one can hardly 

 deny the main thesis. The idea is indeed familiar to those of us who 

 were brought up on Lyell's " Principles " (even the early editions) 

 and on Darwin's " Origin of Species ", but the merit of Dr. Willis 

 is to have made it more precise by a variety of numerical state- 

 ments. He goes, however, much further than this, in that from the 

 universality of his rule he draws the conclusion that the extent of 



