TEE ZOOLOGY OF TO-DAY 253
These and nnmeroxis other experiments (such as those of Schtibeler on German wheat transplanted to Norway and back again^ the work of Tower on the potato beetle, that of Sumner on breeding mice at low and high temperatures, etc.) unquestionably show that the environment can exert a modif3ring influence on the hereditary constitution of a race, that it can originate heritable differences between organisms. They show, moreover, that it sometimes happens that a definite change is made in the body and a corresponding change in the germ cells, the change in the body of the first generation, thus showing in a measure what the heritable effect on the race will be. These important experi- ments mark a real advance, and it is safe to predict that they are but the precursors of many more dealing with the effect of thiB environment on tbe germ cells. At present one can not but feel that the amount of evidence is too slim to decide the question as to whether the environment first produces an effect on the body which is then transmitted to the ' germ cells, or whether the environment acts directly upon the germ cells, producing in them potential changes parallel to those produced in the body.
A second vray in which heritable differences between organisms origi- nate, -Qiat is, a second way in which changes in the properties of the germ cells are induced, is ihrough amphimixis or development from two parents, wherein two sets of hereditary tendencies are intermingled.
Adopting this general method, investigators have in recent years attacked the problems of heredity and variation from two sides. On the one hand, students of experimental embryology, cross-fertilizing the egg of one species with the sperm of another, have occupied themselves in tracing the influence of the respective parents as displayed in the growth and differentiation of the hybrid germ. Sea urchins, frogs, flsh are the objects which more than others have been used for such studies. This is too technical a fleld to admit of brief description. If there were time it would be easy to show that the connections between the study of embryology and heredity are numerous, close, and indeed fundamental to any real understanding of either.
The other great application of the method of cross-breeding to the study of heredity concerns itself not with the gradual individual devel- opment but with the reappearance of the characteristics of adidt organ- isms in the offspring and later descendants. In this study a remarkable activity now reigns, dating from the year 1900, when certain principles of hereditary transmission, originally discovered by Mendel and pub- lished in 1865 but subsequently lost sight of, were rediscovered by sev- eral European botanists. These principles lie at the center of that col- lection of data, law and explanatory hypothesis which we designate Mendelism and which is the outcome of a vast amount of experimental breeding of animals and plants of many kinds.
The fxmdamental principles of Mendelism are no doubt familiar to
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