ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
619 
increase in the depth of water, the Algae especially being never abundant, 
except in shallow (2-10 metres) water. More careful observations 
showed, however, that this is only true in summer, and that the relative 
size of the drainage area of the lake is an important factor. Where a 
shallow lake has large and rapid feeders, its plankton will be less than 
that of a deeper lake w T hich drains a proportionately smaller area. Such 
facts point to the special importance of the temperature of the water and 
of the relative frequency of its renewal. As might be expected, it was 
found that the number of fish in a lake bears a direct relation to the 
amount of its plankton. The paper is illustrated by some very instructive 
tables. 
Evolution of Spines.* — Mr. C. E. Beecher discusses this subject 
with much wealth of example and illustration, his object being to reach 
some general conclusion as to the origin of the spinose condition. Spines 
are purely organic structures, and therefore subject to the law of varia- 
tion ; variation may be either progressive or regressive, and is therefore 
produced either by constructive or destructive agencies. In either case 
the agent may be external stimuli acting on the organism, or growth- 
force acting from within. There are, therefore, four factors concerned 
in the production of spines. But each of these factors is complex and 
capable of subdivision, and there may be a replacement or overlapping 
of forces, so that there are in all eleven causes of spinosity. It may 
arise : — 
“ (1) In response to environmental stimuli acting on exposed parts. 
(2) As the extreme result of progressive differentiation of previous 
structures. 
(3) Secondarily, as a means of protection and offence. 
(4) Secondarily, from sexual selection. 
(5) Secondarily, from mimetic influences. 
(6) From j>rolonged development under conditions favourable for 
multiplication. 
(7) By repetition. 
(8) By restraint of environment causing suppression of structures. 
(9) By mechanical restraint. 
(10) By disuse. 
(11) By intrinsic suppression of structures and functions. 5 ’ 
A detailed discussion of these factors shows that two main generali- 
sations may be drawn up : — “ (1) That spinosity represents the limit 
of morphological variation, appearing in phylogenv as the limit of mor- 
phological progress and regress ; and (2) that similarly, physiologically, 
spinosity represents the paracme of vitality.” In other words, “ after 
attaining the limit of spine differentiation, spinose organisms leave no 
descendants, and from such spinose types no new types are developed.” 
Relations between Marine Animal and Vegetable Life.f — Mr. 
H. M. Vernon, following up his w r ork on the ‘ Effects of Environment on 
the Development of Echinoderm Larvae,’ has made a series of researches 
into the relations between marine animal and vegetable life. His work 
concerns itself with (1) the purification of water by vegetable life, and 
* Amer. Journ. Sci., vi. (1898) pp. 1-20, 125-36, 249-68, 329-59 (1 pi., 73 figs.). 
t MT. Zool. Stat. Neapel, 1898, pp. 341-425. 
