ORGANIC EVOLUTION—MACNAMARA, 875 
The advent of these plants in the flora of this period, according to 
existing fossils, appears somewhat sudden, so much so that palaco- 
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 ‘‘which 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.”* 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 actualspecies. The following 
details concerning a remarkable series of variations in certain Fox- 
1 Ancient Plants, by M. C. Stopes, p. 108, 
