TEE ZOOLOGY OF TO-DAT 257
oontmually escape from its tyranny through modification of their germ< cell Bobfltance, modification which is brought about through interaction with the environment and through interaction with other germ-cell substanceBy the latter action leading not only to new combinations of the old^ as in ideally strict Mendelism^ but to actu^ change in the specific protoplasm^ with the result that what are virtually new qualities emerge.
Mendelism has enormously increased the general interest in hered- ity, than which no subject in the whole field of science is more dis- cussed to-day. In the midst of the discussions and admirable investi- gations dealing directly with this matter^ it is well not to forget what heredity is. As Haeckel pointed out long ago^ heredity is not a special organic function^ but is only a name for the fact that the specific sub- stance of the germ cell exhibits a set of properties substantially like those of the parent germ cell. In other words, heredity means that an egg behaves very much as the parent egg did, because, having essentially the same organization, it reacts to stimuli in essentially the same fashion. A sound knowledge of heredity is therefore dependent on a knowledge of the ways in which the many kinds of protoplasm respond to stimuli; in other words it is dependent on the general level of biological science.
In conclusion, let me say that the several aspects of zoology at which we have glanced has each an interest in itself. Otherwise there would be no hope of advance. But they fade into one another. The data overlap and the problems merge. The geographical explorer, dealing with the distribution of animals; the classifier, discovering and ar- ranging the diagnostic features of races and species; the descriptive anatomist skillfully tracing out details of structure in finished product and embryo; the comparative morphologist, outlining embryological sketches and life histories and applying his data to questions of evolu- tion; the analytic embryologist, unraveling physiological factors, con- trol of which enables him to bring into being the differences which he started out to explain; the student of hereditary transmission recording the way in which characters reappear, and his other half, the student of variation, who experimentally induces new differences — ^these and many others are all dealing with one and the same nature, the many-sided world of living and once living things of which we form a part. The various classes of phenomena exhibited by this world of organisms, as they are mapped out and in some degree analysed, enter into and con- stitute biology. They form a vast and heterogeneous array, of which it may be said that the vastness will remain, will indeed steadily in- crease, but the heterogeneity should become less evident. For as knowl- edge grows and hypothesis gives way to generalization, the various aspects of the living world will no doubt arrange themselves in a more and more coherent manner, that is, we shall be more and more able to assign them to empirically learned causes, to the fundamental powers of the group of protoplasms as shown in responses to stimuli.
VOL. nL— 18.
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