ORGANIC EVOLUTION — MACNAMARA. 375 



The advent of these plants in the flora of this period, according to 

 existing fossils, appears somewhat sudden, so much so that palaeo- 

 botanists have been disposed to think that this epoch indicates the 

 existence of a mutative period in plant life. In fact, that during 

 the time the chalk rocks were forming" that plants suddenly all 

 over the world produced species differing essentially from those 

 which had preceded them. It is necessary, however, to take 

 into consideration the existence of a group of fossil plants known 

 as ' Cycads, which were probably derived from a common stock 

 and " winch are in close connection with the ancestors of 

 modern flowering plants; thus flowering plants can be linked on 

 to the series that runs through the Cycads directly to the primitive 

 ferns." 1 It is only within the last few years that the important 

 extinct group of plants — Pteridosperms — has been recognized. 

 Nevertheless, they form the most numerous plants of the Carbon- 

 iferous period and have displaced the ferns from the position they 

 were hitherto supposed to hold as the dominant plants of the coal 

 measures. Facts such as these render us cautious in accepting the 

 idea that the flowerless flora of the ancient world became suddenly 

 changed during the Cretaceous epoch into flowering plants. It is 

 clear that the vascular and reproductive organs of the plants of 

 ancient geological periods, as they grew taller and came to inhabit 

 a dry soil, must, under the laws of natural selection, have undergone 

 certain modifications. From the microscopical examination of the 

 tissues forming these primitive plants we find that alterations in their 

 structure have gradually taken place, culminating in the appearance 

 of the flowering plants of the Cretaceous epoch. 



Dr. M. C. Stopes in the concluding chapter of his excellent work on 

 "Ancient Plants" (p. 178) states that "the group of fossil plants do 

 not now appear isolated by great unbridged gaps, as they did even 

 20 years ago;" by means of the fossils either direct connections 

 or probable links are discovered which connect series and fami- 

 lies. We may add that plants now growing in the Nile Valley are 

 similar in character to those represented on the monuments of the 

 earliest Egyptian dynasties. In the stable climate and conditions of 

 the Nile Valley these plants for thousands of years have retained 

 their character; but if removed to a different soil and climate such as 

 that of England, in the course of a few generations they become 

 variable and thus undergo marked modifications. We hold this 

 result to be attributable to the response of their living protoplasm to 

 the action of changes of environment, which in the course of time we 

 believe, under the influence of natural selection, might possibly lead 

 to the production of new varieties, if not actual species. The following 

 details concerning a remarkable series of variations in certain Fox- 



» Ancient Plants, by M. C. Stopes, p. 108. 



