in the orientation of organs has the effect of changing the character of organs. [180] Loeb, J., Untersuchungen zur physiologischen Morphologie der Tiere. II. Organbildung und Wachsthum. Würzburg, 1892. [Illustration: FIG. 38] There are only two ways by which we can account for these influences of gravitation. Either certain substances flow to the lowest level and collecting there induce growth and possibly changes in the character of growth (as in _Antennularia_) or if the cells have elements of different specific gravity the relative position of these elements may possibly change and influence in this way the conditions for growth. The influence of gravitation as well as of contact upon life phenomena are at present little understood. In all these cases of heteromorphosis the original form is not restored. It is needless to say that they are incompatible with the theory of natural selection. The reader will have noticed that in this chapter one term has not been mentioned which is commonly met with in the literature, namely the “wound stimulus.” As the writer had indicated in a former publication,[181] the word “stimulus” is generally used to disguise our ignorance of (and also our lack of interest in) the causes which underlie the phenomena which we investigate. Regeneration very often does not take place near the wound but at some distance from it. But even when the regeneration takes place at the edge of the wound the latter only serves to create conditions for regeneration, and these conditions cannot be expressed by the word “stimulus.” [181] Loeb, J., _Die chemische Entwicklungserregung des tierischen Eies_. Berlin, 1909. While our knowledge of the rôle of the whole in regeneration is incomplete in a great many details it seems that the known facts warrant the statement that the phenomena of regeneration belong as much to the domain of determinism as those of any of the partial phenomena of physiology. CHAPTER VIII DETERMINATION OF SEX, SECONDARY SEXUAL CHARACTERS, AND SEXUAL INSTINCTS _I. The Cytological Basis of Sex Determination_ 1. It is a general fact that both sexes appear in approximately equal numbers, provided a sufficiently large number of cases are examined. This fact has furnished the clue for the discovery of the mechanism which determines the relative number of the two sexes. The honour of having pointed the way to the solution of the problem belongs to McClung.[182] It has been known that certain insects, _e. g._, Hemiptera and Orthoptera, possess two kinds of spermatozoa but only one kind of eggs. The two kinds of spermatozoa differ in regard to a single chromosome, which is either lacking or different in one-half of the spermatozoa. [182] McClung, C. E., “The Accessory Chromosome--Sex Determinant?” _Biol. Bull._, 1902, iii., 43.