418 THE DISTRIBUTION OF ANIMALS AND PLANTS 



contain other creatures that appear to be exclusive of them, 

 that our faith in the theory fails. The only theory of evolu- 

 tion which seems to meet this difficulty is that advanced by 

 Mivart, Leconte, and Saporta, of " critical periods," or periods 

 of rapid introduction of new species alternating with others 

 of comparative inaction. This would much better accord 

 with the apparently rapid introduction of many new forms of 

 life over wide regions at the same period. It would also 

 approach somewhat near, in its manner of stating the problem 

 to be solved, to the theory of " creation by law " as held by 

 the Duke of Argyll, or to what may be regarded as " mediate 

 creation," proceeding in a regular and definite manner, but 

 under laws and forces as yet very imperfectly known, through- 

 out geological time. 



It seems singular, in view of the facts of palaeontology, that 

 evolutionists of the Darwinian school are so wedded to the 

 idea of one introduction only of each form of life, and its 

 subsequent division by variation into different species, as it 

 progressively spreads itself over the globe, or is subjected to 

 different external conditions. It is evident that a little further 

 and very natural extension of their hypothesis would enable 

 them to get rid of many difficulties of time and space. For 

 example, certain Millipedes and Batrachians are first known in 

 the coal formation, and this not in one locality only, but in 

 different and widely separated regions. If they took be- 

 ginning in one place, and spread themselves gradually over 

 the world, this must have required a vast lapse of time more 

 than we can suppose probable. But if, in the coal-formation 

 age, a worm could anywhere change into a Millipede, or a fish 

 into a Batrachian, why might this not have occurred in many 

 places at once ? Again, if the oldest known land snails occur 

 in the coal formation, and we find no more specimens till a 

 much later period, why is it necessary to suppose that these 

 creatures existed in the intervening time, and that the later 



