Fl^OM AX OLD STANDl'OINT. 



101 



age of lakes."* Yet, as in Asia, these beds must have origimilly 

 been laid down on the deposits of earlier seas. 



Those who have read Professor Gregory's recent interesting^ 

 book, The Dead Heart of Australia, will remember that, when 

 alluding on pages 153-4 to Prince Krapotkin's view of a general 

 desiccation, he does not commit himself to an opinion for or 

 against it ; but only observes that a period of universal 

 desiccation is not needed to explain the shrinking of Lake Eyre. 

 Anyone who cares to go farther into this matter will find an 

 abundance of data to elaborate : but it may be at once remarked 

 that it is nothing to the point to urge that there has been no 

 marked alteration m climate during the last two or three 

 thousand vears in vegetated regions lono- familiar to us, such a.< 

 in South Europe. What we have to learn is whether the 

 desiccating centres, that is to say, the desert areas, have been 

 increasing in historic times. If the answer depended on the 

 data supplied by the Asiatic continent, it would be certainly 

 affirmative. 



On the tropical sea-borders of a continent such a progressive 

 desiccation would be indicated by the retreat of the mangroves 

 towards the equator. In fact, if the process is still general, 

 there would be a continuous shrinking of the areas held by 

 mangrove swamps in warm regions. In Chapter xxxii of my 

 work on Plant-JJispersal I have given reasons for the belief 

 that the mangroves of the west coast of South America are 

 retreating towards the equator owing to the advance northward 

 of the arid climatic conditions of the Peruvian sea-border.- 

 There are also some ofrounds for thinking; that within historic 

 times the typical mangroves have withdrawn in AVesiern Asia 

 from the Persian Gulf to the mouth of the Indus.:|: 



The Biological Side of the Differentiaiioii Theori/. 



That, with climate as with plants, the line of the development 

 has been from the general to the special, is a ductiine, 1 imagine, 

 which has been commonly accepted. The principle of the 

 difierentiation of floras in the course of geological periods has 



* Geology, by Chamberlin and Salisbury, iii, 193. 



t According to Sir Martin Conway, as quoted by Prof. Gregory, the 

 progressive desiccation of the southern part of South America is indicated 

 by dwindliug glaciers, disappearing lakes, and by the tiansformation of 

 cultivated areas into regions of ariclity. 



X See a note in Geo(jra]:<lucal Joum d for September, 1903, on a work by 

 Dr. Bretzl dealing with the plants referred to in the account given by 

 Tlieophrastus of Alexander the Great's expedition to India. * 



