398 PART IV. VEGETABLE TAXONOMY. 



plants. For example, in the column headed " Lycopods," the 

 black area shows that the oldest known Lycopods occur in the 

 upper Silurian, that the species culminated in the Devonian and 

 Carboniferous, began to decline in the Triassic, and have con- 

 tinued in greatly diminished numbers until the present. 



The Defective Record. The record of the succession of 

 life, is, as we have seen, a very fragmentary one. Of all the 

 millions of species of plants that must have existed in geologic 

 time, but a very small fraction could be preserved ; (1) because 

 the remains of most plants are very perishable ; (2) because the 

 rocks which afford us the only records of the remote past are 

 mostly marine and lacustrine formations, sedimentary deposits, 

 in which only marine species and the few terrestrial or fresh- 

 water ones, that by accident became washed into the seas and 

 lakes, would stand any chance of preservation ; (3) because of 

 the disturbances and changes to which many rocks, particularly 

 those of the older series, have been subjected, tending to oblite- 

 rate all fossil remains. They have often been displaced by 

 earthquake action, metamorphosed by heat and pressure, disin- 

 tegrated by atmospheric or chemical agencies, or eroded by 

 running water. (4) Because even the imperfect record afforded 

 by the rocks is not continuous ; there are great gaps of time 

 entirely unrepresented by rock formations — leaves, and even 

 whole chapters, missing from the record book. Throughout 

 geologic time periods of submergence, when sediments were 

 deposited and rocks formed, alternate with periods of elevation 

 above sea-level, and when erosion instead of rock-building took 

 place. So far as history is concerned, the periods of elevation, 

 however long they may have continued, are often lost intervals, 

 no record of them remaining save the effects of erosion. 



But even the very fragmentary record we have, clearly shows 

 a progress from lower to higher. Algae were the dominant types 

 of the Silurian ; vascular cryptogams of the Devonian and Car- 

 boniferous ; Conifers and Cycads of the Mesozoic, and Angio- 

 sperms of all subsequent times to the present. It is what might 

 have been expected on the theory of evolution. 



Moreover, it is in accordance with this theory that the types 

 which first appeared were what naturalists call comprehensive or 

 generalized types ; that is, they united in themselves characteris- 



