104 
PRINCIPLES OF BIOLOGY. 
Next come the Silurian strata, and we now meet, for the 
first time, with representatives of the Lower Silurian forma¬ 
tion, in the shape of Caradoc sandstones and shales ; but even 
these are not found north of the Severn, while the Aremg 
Beds and the Llandeilo Flags are still wanting. Neither is 
there any trace of the Lower Llandovery Beds, but the Upper 
Llandovery, or May Hill Sandstone, rests unconformably on 
all the rocks below it, while above it come all the Upper 
Silurians in due order—this is, indeed, their typical district— 
the Woolhope, Wenlock, and Aymestry Limestones forming 
long ridges, running from north-east to south-west (of which 
Wenlock Edge is the most prominent), while the softer shales 
form the valleys between. Then, west of Bridgnorth, and 
south of Broseley, we find the Old Red Sandstone here termina¬ 
ting its north-easterly extension. Of the Lower Carboniferous 
strata the Mountain Limestone and Millstone Grit are but 
feebly developed at Lilleshall and south-east of Wellington ; 
while fifteen miles farther south, in the Brown Clee Hills, 
the Coal-Measures repose upon the Old Red Sandstone. 
(To be continued.) 
THE PRINCIPLES OF BIOLOGY. 
BY HERBERT SPENCER. 
BY LEWIS J. MAJOR. 
ABSTRACT OF CHAPTERS VIII. AND IX., VOL. I. 
Heredity. 
The universal law of Heredity is that each plant or animal 
produces plants or animals of the same general structure 
with itself, and though the various instances of heterogenesis, 
as given in the preceding chapter, are at first sight at 
variance with this law, they are not really so, the recurrence 
of forms being cyclical instead of direct. But the law of 
Heredity applies not only to main characters of structure but 
also to the smaller details, and though the tendency to 
repetition is qualified somewhat by the tendency to variation, 
the law may be considered as unlimited. Thus the members 
of any one sub-species habitually transmit their distinctive 
peculiarities to their descendants. We have, for example, 
several varieties of wheat, many varieties of potatoes and 
peas and of numerous other vegetables, not to speak of 
flowers. 
