108 PROF. ERNEST W. MACBRIDE, M.A., F.R.S., ON 



Mendelians : If the hermit crab acquired its peculiar abdomen by 

 the loss of the factor for straightness, how is it that the young 

 hermit crab has a straight abdomen ? 



Such reasoning as this raises at once two objections ; first, it 

 may be asked, is there any evidence from experiments that such 

 reactions to environment, in a word, such acquired characteristics, 

 can be inherited ? and, secondly, if they can, by what mechanism 

 can this be accomplished ? To answer the first objection we 

 may add that such evidence is difficult to obtain, because to 

 produce it demands experiments carried on over a much longer 

 series of years than any Mendelian has as yet attempted. 

 Nevertheless, in a few cases there is some indication of this 

 inheritance of reaction. In the spotted salamander, for instance, 

 it has been found that if the beast be reared on a dark back- 

 ground the spots of yellow diminish in size, and when this 

 has gone on for several generations the young born, even if reared 

 in normal surroundings, have smaller spots than young born of 

 salamanders which have always lived in normal surroundings. 



A good many cases of the same kind have been recorded from 

 among plants ; and it seems clear that when a plant or animal 

 reacts to new conditions by a change of structure, if the influ- 

 ence of the new conditions continues long enough the change of 

 structure becomes in time hereditary. 



As to how the heredity can become affected, we do not, of course, 

 know, but we can make a guess. We are beginning to know a 

 little of the manner in which the complex body of the higher 

 animal is built up out of the germ. We find at first a few 

 organ-forming substances dispersed in the protoplasm of the 

 germ. By the action of these the first simple tissues are built 

 up. Then these tissues act on each other by emitting chemicals 

 termed hormones. To give an example : if the stalked eye of 

 a shrimp be pulled off, it grows a new one. But if the optic 

 ganglion beneath the eye be removed, then, instead of a new 

 eye, an antenna is produced. The only way to account for this 

 is to assume that under normal circumstances some chemical 

 is emitted by the ganglion which causes the skin above it to 

 mould itself into an eye. 



Xow, if by a reaction to new conditions the tissues of an 

 animal change, they will emit a new type of hormone into 

 the blood, and these hormones will after a time be built 

 up in the genital cells. When these cells develop ihe 

 modified hormone will be set free, and will cause the modifi- 

 cation of the tissues, even before the new environment has 

 time to act. 



