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JOURNAL OF THE PLYMOUTH INSTITUTION. 
up very readily from self-sown or bird-sown seed, as does also 
frequently the North American species, Ribes sanguineum. This 
last, although introduced so comparatively recently as 1826, has 
become one of the commonest of cultivated flowering shrubs, and 
proves a great adornment of our gardens in early spring. 
The local habitat of Tillcea muscosa is for the most part soil 
formed by the disintegration and decomposition of Devonian Slate, 
mixed probably with a small amount of recent vegetable mould, 
the decayed remains of minute Musci. It is too small a species to 
exist amongst dense vegetation, even if only a few inches in height, 
and consequently must be restricted to spots bare of, or unsuited 
to, most other phanerogamous species. In the Plym valley the 
cropping down by rabbits of some of the vegetation around the 
spots where it occurs may be the means of keeping an additional 
amount of surface open to it. Bearing in mind the great fact of 
species competition, enquiries and investigations respecting the 
occurrence of this humble little annual in the Plym valley are 
fraught with interest. The differences in size amongst the indi- 
vidual plants show that the least development is attained by them 
when the stratum of soil in which they grow is of the scantiest and 
driest nature, and that increased luxuriance is in relative proportion 
to an increased depth of earth and supply of moisture. But as the 
soil deepens so does it become increasingly possible for species larger 
than the little Tillwa to exist, with the result of an unequal com- 
petition between the great and the small. It is a positive certainty 
that the spots where we find individual plants of various kinds are 
often not those best adapted to them as species. See a diminutive 
bush of Salix cinerea growing from the crevice of a dry limestone 
wall. We know it would have grown ten times as fast had the 
down-appendaged seed from which it sprang been blown, not into 
the wall, but to an unoccupied spot on the damp side of the ditch 
below. Smith in his interesting English Flora locates the 
Tillcea " on the most barren sandy heaths," and both in the east of 
England and on the Continent it seems to affect such spots. A 
country with a very gradually rising temperature in spring is prob- 
ably best adapted to it, as any sudden occurrence of unusual heat 
or drought is liable to burn it up in its arid habitats before the pro- 
duction of much seed — a casualty which sometimes happens to it 
even in the damp climate of Devon. If it be granted that species 
competition is the force most powerful in restricting the Tillcea to 
