It need hardly be said that what seems to have been made very probable for the pigeon need not hold good in the same way for other animals. Thus in frogs it is probable that all the eggs are alike to begin with, but that there are two kinds of spermatozoa, one with and the other without a special sex- chromosome. If the eggs are quite normal at the time of fertili- zation the determination of the sex depends on whether the egg is fertilized by a sperm with the special sex-chromosome (which results, it is believed, in a female offspring) , or by one without the sex-chromosome (which results, it is believed, in a male offspring). Even clearer cases have been worked out in insects by Prof. E. B. Wilson and others. It does not follow that the physiological condition of the ova is of no importance, for over- ripe frog's ova tend to develop into females. Moreover, the pres- ence of the special sex-chromosome may be associated with certain constitutional differences in the germ-cell an index as much as a cause.
Regenerative Capacity. Prof. T. H. Morgan's critique (1901) of Weismann's theory of regeneration as an adaptive phenomenon occurring in those two animals and in those parts of animals which, in the natural conditions of life, are particularly liable to non-fatal injury, has been followed by interesting experiments and observations. It seems that regeneration often follows the experimental excision of a part which could rarely, if ever, be lost in the ordinary conditions of life. In such cases it seems gratuitous to search for some far-fetched evidence of adaptiveness. It seems more reasonable to regard the regrowth of an excised portion of the body as a special exhibition of a power which is always in operation in the repair of certain tissues, such as the glandular epithelium of an intestine. On the other hand, this will not suffice when there is definite prearrangement establish- ing a weak breaking plane, as in a crab's claw or a lizard's tail. The fine work of Dr. J. H. Paul (1915) has shown in the case of the shore-crab (a) the definite prearrangements at the breaking plane; (b) the muscular arrangements that secure the rapid surrender of a captured or broken limb; and (c) the presence of a two-flapped membrane, pierced by nerve and blood-vessel, which automatically folds over the wound and prevents haemorrhage. To this has to be added the way in which the new limb is fully formed within the scar before it is jerked out after a moult. When it is known that a breakage of a leg by a dislodged stone is a common accident among shore-crabs, the adaptiveness of the special arrangements seems more than plausible.
Senescence and Rejuvenescence. The study of development in the widest sense includes the phenomena of senescence as well as those of adolescence, and there is no doubt that progress is marked by such studies as those of C. M. Child (1915). On an experimental basis, mostly in regard to planarian worms, Child rests the conclusion that as an organism differentiates, it ages, for there is a necessary accumulation of relatively inactive and stable constituents in the colloidal cytoplasmic substratum, and this accumulation involves a decrease in the rate of metabolism, as the flow of a stream may be slowed by what it deposits in its bed. There are indeed counteractive processes of reduction, re- moval, and de-differentiation, when the metabolic stream may be said to erode its bed instead of depositing more materials. Periods or it may be crises (often seasonal) of rejuvenescence alternate with periods of senescence. Sooner or later, however, rejuvenescence lags and senescence prevails. Exceptions, as Weismann and others suggested, are to be found in the Protozoa, where recuperative rejuvenescence is wellnigh so perfect that it implies bodily immortality. The question rises whether a similar evasion of natural death may not occur also in some simple Metazoa such as hydroid polyps and planarian worms.
In the same connexion may be noticed the observations of L. L. Woodruff on pure-line (all descended from one individual) cultures, of the slipper-animalcule Paramoecium, which multi- plies rapidly by fission. In 1915 he reported on a stock which had kept agoing for over eight years, 5,250 or more asexual generations. There had been no conjugation (which does not occur between members of a pure line) and no artificial stimu- lation; the secret of continued vitality, so different from the
aging observed by Maupas, being abundant food and a thorough removal of waste products from the water. Very suggestive is the periodic, approximately monthly, occurrence of endomixis observed by Woodruff and Erdmann (1914), a disruption and reorganization of the nuclear apparatus, similar to that which precedes conjugation in natural conditions. It appears to serve as a process of rejuvenescence, and it serves, perhaps, like con- jugation, to provoke variability.
Another correlated idea, to which the work of some palaeontol- ogists has pointed, is that one type of animal may differ from another in the relative length of the chapters in the life-history. The pre-natal chapter may be long in one, e.g. elephant, and very short in another, e.g. opossum; the larval period, long drawn out in a crab, is telescoped down in the crayfish. The young storm-petrel remains many weeks in the burrow; the young mound-bird may run or fly from its birthplace on the day it is hatched. A queen-ant or a queen-bee may live for several years as an adult, while the may-fly sometimes completes its adult We in an evening. The idea is that an arc in the curve of life may be shortened or lengthened in related types, as the adaptive outcome of " temporal " variations. In vertebrate animals it is known that changes in the activity of the glands of internal secretion may hasten or slow down processes of development as well as processes of metabolism.
(D) AETIOLOGICAL EVOLUTIONARY ZOOLOGY Heredity. Prof. T. H. Morgan (1915, p. 7) calls attention to a curious situation which has begun to develop since 1900, when Mendel's law was rediscovered. " The students of heredity, calling themselves geneticists, have begun to draw away from the traditional fields of zoology and botany, and have con- centrated their attention on the study of Mendel's principles and their later developments. The results of these investigators appear largely in special journals. Their terminology is often regarded by other zoologists as something barbarous outside the ordinary routine of their profession. The tendency is to regard genetics as a subject for specialists instead of an all- important theme of zoology and botany." It is to be hoped that this severance is only a passing phase, for its accentuation would mean much loss on both sides. " It would be as unfortunate for all biologists to remain ignorant of the modern advances in the study of heredity as it would be for geneticists to remain uncon- cerned as to the value for their own work of many special fields of biological inquiry." Of clear exposition by investigators like Bateson and Punnett, Castle and Morgan, there is no lack, and the advantages of a synoptic view of the fundamentals of zoology as a whole do not need to be enforced by argument.
Referring for due treatment to the article MENDELISM, we may call attention here to three points of general interest. (a) In two recently published books we find two extraordinarily discrepant statements in regard to heredity. In one of them Prof. J. P. Lotsy (1916, p. 63) says: " We know absolutely nothing of heredity." In the other Prof. T. H. Morgan (1919, p. 15) speaks of the fundamental aspects of heredity having turned out " so extraordinarily simple," and he elsewhere in- dicates that the problem of heredity may be regarded as solved. What is the meaning of this startling divergence of statement? The first author had in mind the difficulty of conceiving how all the manifoldness of a well-endowed organism is telescoped down into a microscopic implicit individuality the germ-cell. The second author was contemplating the precise way in which certain characters of the parents, carried by the chromosomes, are distributed among their offspring. For a large number of cases the modern theory of factors or " genes," located in linear order in the chromosomes, is convincing and clear, (b) The list of characters proved to exhibit Mendelian inheritance is always increasing. Many organisms consist, in part at least, of a great bundle of " unit-characters," the factors for which behave in inheritance as if they were indivisible entities. They do not blend or intergrade; they are present in a .certain proportion of the progeny; they are normally either there in their entirety or completely absent. It is now recognized, indeed, that one
