tional activity of growth and development will go on normally at the expense of change of morphological form.

Normal Growth.-This explanation assumes that there is a normal growth, and the determining of what is normal to each individual is found in the ancestry; i.e., at the outset of embryonic growth the normal function of the development of the individual is already determined. This includes the attainment of the morphological and the physiological characters of the class, order, family, genus, and species to which the organism belongs. The egg at the first appearance of the embryo is determined not only to be a vertebrate, but a bird, of the order Rasores, of the suborder Gallinæ, of the family Phasianidæ, of the genus Gallus, and of one of the many varieties of the species Gallus domesticus. Such is the normal development for that particular embryo. The laws of the development in its every step may be studied, and have been. very fully traced in this particular case, and the knowledge of the law is based upon the observed order of these steps in the development; the inference which we naturally draw is that every new development of a similar egg will be the same.

Natural Selection, as an explanation of the changes which transpire in phylogenesis, assumes that the slight adjustments of the morphological characters, which take place in the ontogenesis of the individual, are added to the determining factors of development for the next generation; that adjustments which are very slight in each case, by accumulation from generation to generation, bring about the differences which distinguish the various species, genera, families and orders of the classes of the animal kingdom. And this is what is meant by "descent with modification." Instead of the idea of descent along a uniform line, in which the offspring differs in only unimportant and strictly variable characters from any of its ancestors, the school of Darwin holds that the slight variations observed (between the offspring and parent, or among the offspring of a common parentage) do not tend to become less in succeeding generations, but that the variations have unequal values in relation to the advantage of the individual; and in the struggle of individuals for life, those individuals possessing the slightest advantage over their fel

lows will, in the long-run, survive them in the race, and they will increase and prevail while the others will drop out and be lost.

Definition of Ontogeny and Phylogeny. In the analysis of Huxley's definition of evolution (or development) the twofold division of the history is adopted, which is expressed in part by the terms Ontogenesis and Phylogenesis, introduced by Haeckel. Haeckel briefly defined these terms, as follows: Ontogeny, or Ontogenesis: The history of the development of the individual (including Embryology and Metamorphology); Phylogeny, or Phylogenesis: The paleontological history of the development of the ancestors of a living form. It is proposed to restrict the term development to the meaning expressed by Ontogenesis, and to restrict the use of evolution to Phylogenesis. In Ontogeny we find the individual organism beginning with a great majority of its lines of development or steps of metamorphism already determined for it. Take, as Take, as an example, the crayfish, which Huxley has so interestingly dissected and described,* and of it we can say at the first stage of the embryo that in case it lives. at all, whatever the conditions of environment may be, it will develop all the characters of the branch, class, order, family, genus, and species to which it belongs. Its name, Astacus fluviatilis, applies to it in all stages of its development from the embryo up (Fig. 50).

The Main Features of Development Predetermined before they Begin. We can predict before any trace of the characters. appear (with as great a degree of certainty as we can predict the result of combining a given acid with a grain of chemical salt) what the path of development will be which the embryo will take if it continues to grow. It will surely develop a jointed body, with the articulated limbs and chitinous crust of the Arthropoda. It will surely develop a breathing apparatus of gills situated on the maxilliped and legs of the Crustacea. The appendages of the cephalothorax will certainly be antennæ, and the specialized biting mouth parts of the sub-class Neocaridæ, not the simple legs of the more ancient sub-class

*The Crayfish, an Introduction to the Study of Zoology" (Appletons, 1880).

Paleocarida; it will have the twenty segments, the specialized carapace, the pair of mandibles, the two pairs of locked maxillæ, and other characters of the order Decapoda, and all the peculiarities of the family Potamobiidæ will be strictly carried out. These concern the whole of the morphology, but in some characters of still less importance the certainty is not so great. This individual will develop on the first somite or ring of the abdomen small appendages,-certainly if it be a male, and exceptionally if it be a female,-whereas, if its

FIG. 50. Astacus fluviatilis. Side view of a male specimen (nat. size). bg, branchiostegite; cg, cervical groove; r, rostrum; t, telson; 1, eye-stalk; 2, antennule; 3, antenna; 9, external maxillipede; to, forceps; 14 last ambulatory leg; 17, third abdominal appendage; xv, the first and xr the last abdominal somite. (After Huxley.)

ancestors had been the closely allied Parastacidæ, no appendages would be developed. Again, in all the details of structure of parts it will be a true Astacus, and not a Cambarus, closely allied genus; and finally, if it were taken to California, and placed under identical conditions with the native Astacus nigrescens, it would still differ in all its specific characters from that species-characters which consist mainly in differences of form and proportion of the parts, which are in number, structure, and function the same for the two species.

Slight Possible Effect of Environment.-Environment might produce slight modification in some of its very insignificant characters, but otherwise its total anatomy and physiology

were predetermined when it began its development. So it is with all organisms. It was this fact, of the perfect repetition of all the essential characters of the ancestors in the new individual, that seemed, in the minds of the early naturalists, so absolutely to fortify the belief in the immutability of species. The slight modifications in unimportant details. appeared as mere accidental imperfections of the individual. But it was in these slight variations that Darwin found the secret of evolution.

CHAPTER X.

WHAT IS THE ORIGIN OF SPECIES? THE PROBLEM AND ITS EXPLANATION.

We have seen that there are organic individuals; that they all, however complex their organization, begin as simple cells, and pass through, in each case, definite stages of development, assuming by degrees greater and greater differentiation of the cell. The chief stages of this development are the cellular segmentation, the formation of tissues in embryonic growth, and the attainment of maturity by steps of modification which are in almost every observable particular the exact repetition of steps of modification which their immediate parents passed through in attaining their maturity.

Variation and Mutability Essential Presumptions in the Discussion of Origin of Species.-The differences which the individual presents, when closely compared with its parents, are called variations, or varietal characters. The characters which each individual possesses in common with its parents are classified according to their importance and permanence, and arranged in order from lesser to greater, as specific, generic, family, ordinal, class, or branch characters.

It is a generally accepted belief that the assumption by the individual of all of the characters which it bears in common with its immediate ancestors is sufficiently accounted for by what are called the natural laws of reproduction; that the slight departure from exact repetition is an insignificant and indeterminate accident of all organisms, or that it is an expression of the imperfection with which the process of reproduction acts.

The theory of zoologists of the first half of the century was that the species were immutable; that variations were not cumulative, but were always simply variations, the spe