THE SEVEN AGES OF PLANT LIFE ^5 



Now the actual length of these various peridds was 

 very different. The epoch of the Present Day is only 

 in its commencement, and is like a thin line if compared 

 with the broad bands of the past epochs. By far the 

 greatest of the periods is the Archaean, and even the 

 Older Palaeozoic is probably longer than all the others 

 taken together. It is, however, so remote, and the rocks 

 which were formed in it retain so little plant structure 

 that is decipherable, so few specimens which are more 

 than mere fragments, that we know very litde about it 

 from the point of view of the plant life of the time. 

 It includes the immense indefinite epochs when plants 

 began to evolve, and the later ones when animals of 

 many kinds flourished, and when plants, too, were of 

 "great size and importance, though we are ignorant of 

 their structure. Of all the seven divisions of time, we 

 cafi say least about the two earliest, simply for want of 

 anything to say which is founded on fact rather than on 

 theoretical conclusions. 



Although these periods seem clearly marked off from 

 one another when looked at from a great distance, they 

 are, of course, but arbitrary divisions of one long, con- 

 tinuous series of slow changes. It is not in the way of 

 nature to make an abrupt change and suddenly shut off 

 one period — be it a day or an aeon — from another, arid 

 just as the seasons glide almost imperceptibly into one 

 another, so did the great periods of the past. Thus, 

 though there is a strong and very evident contrast be- 

 tween the plants typical of the Carboniferous period 

 and of the Mesozoic, those of the Permian are to some 

 extent intermediate, and between the beginning of the 

 Permian and the end of the Carboniferous — if judged by 

 the flora — it is often hard to decide. 



It must be realized that almost any given spot of 

 land^— the north of England, for example — has been 

 beneath the sea, and again elevated into the air, at 

 least more than once. That the hard rocks which make 

 its present-day hills have been built up from the silt 



