Two Recent Ecological Papers. 
22 1 
and till we have more evidence than is at present available of the 
possibility of arriving at these critical factors with some degree of 
exactness, the quantitative investigation of the habitat must be 
more or less on its trial. 
The second part of the paper is devoted to a consideration of 
the relations of neighbouring formations to the one which is the 
main subject of the author’s investigation, and many interesting 
data regarding invasion and succession are given. It seems that 
the pine formations of the mountains are advancing eastwards. 
The weak points of the paper seem to be the absence of any 
special study of the natural history of the dominant plants, of the 
details of means of dispersal and competition, as well as the failure 
to analyse the physical factor data so as to determine the limiting 
conditions of the existence of the different types. In order to push 
the analysis of the vegetation to its fundamental factors such study 
would seem to be essential, and no ecological study that we have 
yet seen has carried analysis to such a point. Nevertheless what 
has been done in this case has evidently involved an immense amount 
of valuable work, and we have to thank the author for a picture 
of a plant-formation completer in some respects than we have yet 
obtained. The paper is illustrated by several good half-tones, 
besides the numerous tables. 
Dr. Woodhead’s paper originated in a desire to study more 
closely part of the area included in the general survey by Smith and 
Moss of the Leeds and Halifax district of Yorkshire. He chose an 
area of about 66 square miles immediately to the south of 
Huddersfield, extending from the coal measure shales on the east, 
to the high moors of the Pennines on the west, and including many 
woods. 
The primary method adopted was to map on a large scale the 
distribution of the principal ground plants, such as the Bracken and 
the Bluebell, of a small woodland area and to correlate these 
distributions with those of the shade trees and the soils. In this way 
a very clear relation between the soil and shade on the one hand and 
the ground flora on the other was brought out. In a given wood, while 
the Bracken grew in all the soils present under oak, it was excluded 
by the deeper shade of elm, sycamore, and beech, while the Bluebell 
depended upon humus for dominance, and thinned out or disappeared 
over bare sandstone and clay. Another fact brought out in the same 
woods is the existence of a “ complementary association ” of Scilla, 
Pteris and Holcus mollis, whose underground parts occupy different 
levels in the soil, while the aerial parts flourish largely at different 
