LIFE IN PRIMARY PERIOD 203 



Papaveracese, Berberidaceae, etc. It is rather astonishing that 

 there are not more of these plants among the fossils of the 

 Secondary Period, but we shall find other lacunae in the Animal 

 Kingdom that indicate quite clearly the paucity of the available 

 evidence. From the beginning of Tertiary times, all the 

 present-day plant-types can be traced. Their distribution, 

 however, is different, and we have seen how important is the 

 study of their geographical distribution to our knowledge of 

 climate in different parts of the earth. We need not go over 

 this again. 



In the preceding pages (p. 109) we have given our reasons for 

 considering the Monocotyledons as being derived from Dicotyle- 

 dons living in marshy soil, and which owe to their ordinary 

 habitat in regions of this nature, their thick parallel-veined 

 leaves, their long underground stems, or their bulbs, and the 

 peculiar structure of their aerial stem, which resembles that 

 of the vascular Cryptogams, whose stem is likewise often 

 developed on the underground stems or rhizomes. Van 

 Tieghem has established, moreover, that the Graminaceae at 

 least, which appeared rather late, are Dicotyledons in which 

 one of the cotyledons has been suppressed. They must have 

 appeared after the Dicotyledons ; but it is impossible to 

 establish the exact date of the appearance of either. In any 

 case they became sharply differentiated only at the time when 

 the Dicotyledons were already numerous in the Cretaceous 

 deposits, and since the Dicotyledons probably go back as 

 far as the Jurassic, the appearance of the Monocotyledons 

 in the course of the Cretaceous is in no way astonishing. 



Theory and fact are, therefore, in perfect agreement. More- 

 over, as the laws of tachygenesis apply equally well to the 

 Animal as to the Vegetable Kingdom, we may have confidence 

 in the inductions we are going to draw from it as regards the 

 first of these kingdoms. 



Theory demands for animals as logical an order of evolution 

 as for plants, but their variety is much greater. The unicellular 

 animal organisms called Protozoa, which constitute the first 

 step in animal organization, ought to have appeared first. 

 But we can hardly expect to find any traces of these delicate 

 organisms, as e.g. Rhizopods, with their diffluent protoplasm 

 incessantly changing form, either by sending out delicate 

 ramifications that often anastomose their network into still 



