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SCIENCE 



[N. S. Vol. XXVII. No. 681 



proaelied from many sides— that of the 

 breeder, the experimenter, the statistician, 

 the physiologist, the embryologist, the 

 cytologist— but the mechanism of heredity 

 can be studied best by the investigation of 

 the germ cells and their development. Al- 

 though many phenomena of inheritance 

 may be discovered in the absence of any 

 definite knowledge of the mechanism of in- 

 heritance, a scientific explanation of these 

 phenomena and practical control over them 

 must wait upon our knowledge of their 

 causes. Only a beginning has been made 

 in this study and it seems probable that it 

 may engage the attention of many future 

 generations of biologists, but, nevertheless, 

 we have come far since that day, only 

 about thirty years ago, when Oscar Hert- 

 wig first saw the approach and union of 

 the egg and sperm nuclei. Indeed, so 

 rapid has been the advance of our knowl- 

 edge within this field that it is hard to 

 realize that this entire period lies well 

 within the' lifetime of most of those here 

 present, and that many of the pioneers in 

 this work are still active in research. 



In the short time at my disposal I can 

 not present even the briefest summary of 

 the many important discoveries in this 

 field; I can hope only to discuss a few of 

 the most suggestive facts and theories as 

 to the mechanism of heredity. 



Heredity, which originally meant heir- 

 ship, ot the transmission of property from 

 parents to children, has come to mean "the 

 transmission of qualities or characteristics, 

 mental or physical, from parents to off- 

 spring" (Century Dictionary). These 

 qualities are frequently regarded as inde- 

 pendent entities or ' ' unit qualities, ' ' which 

 are "transmitted" from one generation to 

 the next through the agency of an "in- 

 heritance material" or "bearer of hered- 

 ity." Such terms are not without fault 



Avhen used merely as figures of speech, but 

 when interpreted literally, as they fre- 

 quently are, they are altogether mislead- 

 ing; they are the result of reasoning about 

 names rather than facts, of getting far 

 from phenomena and philosophizing about 

 them. The comparison of heredity to the 

 transmission of property from parents to 

 children has produced confusion in the 

 scientific as well as in the popular mind. 



It is only necessary to recall the most ele- 

 mentary facts about development to recog- 

 nize that in a literal sense parental char- 

 actei'istics are never transmitted to chil- 

 dren. Every new individual is only a de- 

 tached portion of an old one, and whether 

 derived as bud, or spore, or egg, it owes 

 its similarity to its parent to the fact that 

 it was once a part of it, and not to some- 

 thing which has been "transmitted" from 

 one generation to the next. Furthermore, 

 from its earliest to its latest stage an in- 

 dividual is one and the same organism ; the 

 egg of a frog is a frog in an early stage 

 of development and the characteristics of 

 the adult frog develop out of the egg, but 

 are not transmitted through it by some 

 "bearer of heredity." 



Indeed, heredity is not a peculiar or 

 unique principle for it is only similarity of 

 growth and differentiation in successive 

 generations. The fertilized egg cell under- 

 goes a certain form of cleavage and gives 

 rise to cells of particular size and struc- 

 ture, and step by step these are converted 

 into a certain type of blastula, gastrula, 

 larva and adult. In fact, the whole 

 process of development is one of growth 

 and differentiation, and similarity of these 

 in parents and offspring constitutes heredi- 

 tary likeness. The causes of heredity are 

 thus reduced to the causes of the succes- 

 sive differentiations of development, and 

 the mechanism of heredity is merely the 

 mechanism of differentiation. The pecul- 

 iarity which distinguishes the differentia- 



