ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
515 
different bearers, pangenes, idioblasts, or biophors, as the hypothetical 
particles are variously called. 
Quetelet observed (1870) that the variations of a single character are 
symmetrically grouped around “ a centre of greatest density.” Galton, 
Weldon, and others have increased our knowledge as to the laws of 
these variations, and de Vries has for years accumulated relevant facts 
as to (Enothera Lamarkiana , Heliantlius annuus, Coreopsis tinctoria, 
Anethum graveolens , Caltha palustris , and many other flowers whose 
variations illustrate the phenomenon known as Galton’s half curves. 
These he regards as indices of a discontinuous saltatory variation which 
itself varies continuously. 
De Vries thinks he has experimental evidence to prove the trans- 
formation of a unilateral variation into a symmetrical variation. The 
summit of the new curve does not coincide with the normal character 
of the species, but depends on the novel character of the variety. Thus, 
in regard to the number of petals, the half curve of Galton observed in 
De Vries’ cultures does not result from a fluctuating variation in the 
number, but from a brusque doubling, which again is in itself variable. 
The observed variation, continuous in appearance, is in reality dis- 
continuous. 
Stimuli considered in Relation to Development.* — Herr Curt 
Herbst distinguishes “ directive stimuli ” — light, gravity, chcmotaxis, 
heat, electricity, currents, &c. — and formative stimuli, such as the 
contact of one body on another. He gives a valuable summary of facts, 
to which he has himself added, discusses the general physiology of 
stimulus and reaction, and applies the results to the interpretation of 
developmental processes. Thus he discusses the aero (oxygeno)-taxis 
of the segmentation cells in Arthropod ova ; the chemo (troplio)-taxis 
of yolk-cells, the cause of the origin of Schwann’s sheath and neurilem- 
mata, the origin of connective and muscular sheaths around vessels, the 
origin of other connective-tissue sheaths, the origin of the Polyclad-gut, 
the directive stimuli in the development of freshwater Turbellarians, and 
so on. As a contribution to a rational ontogeny Herr Herbst’s paper is 
of great interest and importance. 
Theory of Development.! —Dr. W. Haacke, continuing his copious 
criticisms of modern theories of heredity and development, works out 
four examples to show that Weismann’s theory of determinants leads 
to the old preformationist theory of “ emboitement ” — the Einschacli- 
telungs-theorie, or SJcatulations-theorie, as Haeckel calls it. And this, 
somehow or other, leads to a creationist dogma, which is a reductio ad 
absurdum ; Haacke is equally severe on Hertwig, who falls, he says, 
into the same error as W'eismann, in reflecting back on the Anlage the 
high organisation of the adult. 
Heredity and Development ! — Dr. W. Haacke continues his vigorous 
criticisms. It is neither necessary nor justifiable to postulate a highly 
organised Anlage for a highly organised organism. The human ovum 
may be as low in organisation as an Amoeba. Specification of cells is a 
function of the position which they occupy in the organism ; the endo- 
* Biol. Centralbl., xiv. (1894) pp. 657-66, 691-7, 727-44, 753-71, 800-10. 
t Op, cit., xv. (1895) pp. 481-91. t_Op. cit., pp. 536-56. 
