18 



The Journal of Heredity 



that these variations are inherited in a 

 perfectly definite and ])redictable way, 

 in accordance with the laws of Mendel. 



If the problem of the origin of varia- 

 tions has not actually been solved, we 

 at least have reached a much more exact 

 comprehension of it. We have learned 

 that the animal's own activities can 

 not be invoked to account for varia- 

 tions, and that the environment's direct 

 action cannot exi)lain them. We have 

 seen them to be due to changes in the 

 struct vire of the germ-cell. It is not 

 certain that we can ] produce experi- 

 mentally a single inherited variation — 

 our knowledge is, therefore, much lack- 

 ing ; but the investigators can certainly 

 "report progress," even though it be 

 mainly of a negative kind. 



If we turn to the second of the great 

 problems, namely, the way in which 

 variations arc transmitted, the nature of 

 the mechanism of heredity, we get a more 

 favorable report. Here are Dr. Mor- 

 gan's own words : 



"the problem of heredity 



' ' I have passed in review a long 

 series of researches as to the nature of 

 the hereditary material. We have, in 

 consequence of this work, arrived within 

 sight of a result that a few years ago 

 seemed far beyond our reach. The 

 mechanism of heredity has,. I think, 

 been discovered — discovered not by a 

 flash of intuition but as the result of 

 patient and careful study of the evi- 

 dence itself. 



"With the discovery of this mec- 

 hanism I venture the o])inion that the 

 ]jroljlem of heredity has Ijcen solved. 

 We know how the factors carried In' the 

 parent are sorted out to the germ cells. 

 The explanation does not pretend to 

 state how factors arise or how they 

 influence the development of the em- 

 l)ryo. But these have never been an 

 integral ijart of the doctrine of heredity. 

 The jjroljlems which they i)resent must 

 be worked out in their own field. So, I 

 rejjeat, the mechanism of the chromo- 

 somes offers a satisfactory solution of 

 the traditional ];roblem of heredity." 



It is not necessary here to descriljc 



this mechanism in the germ-cells, for 

 an account of Dr. Morgan's work on it 

 was presented in the Journal of 

 Heredity only a few months ago.^ 

 The germ-cells contain little rods of 

 easih' stained material to which the 

 name of chromosomes has been given, 

 and changes that occur in the com- 

 position of these rods seem to result in 

 the end-effects which are seen in char- 

 acters of the animal or plant. 



In his last chapter. Dr. Morgan re- 

 turns again to the doctrine of natural 

 selection, Darwin's principal contribu- 

 tion to the ]jrol3lem of finding how 

 evolution takes place. "In his great 

 book on the Origin of Species Darwin 

 tried to do two things: first, to show 

 that the evidence bearing on evolution 

 makes that explanation {i. e., evolution) 

 probable. No such great body of evi- 

 dence had ever been brought together 

 before, and it wrought, as we all know, 

 a revolution in our modes of thinking. 



"Darwin, also, set himself the task 

 of showing how evolution might have 

 taken place. He pointed to the in- 

 fluence of the environment, to the 

 effects of use and disuse, and to natural 

 selection. It is to the last theory that 

 his name is especially attached. He 

 appealed to a fact familiar to every one, 

 that no two individuals are identical 

 and that some of the differences that 

 the\- show arc inherited. He argued 

 that those individuals that are best 

 suited to their environment are the 

 most i^robable ones to survive and leave 

 oft"s])ring. In consequence their des- 

 cendants should in time replace through 

 com]jetition the less well-adajjtcd indi- 

 viduals of the si)tcies. This is the 

 process Darwin called natural selection 

 and S])L'ncer, the sur\-i\-al of the tiltest." 



OBJECTIONS TO DARWIN's VIEWS 



The objections to it. Dr. Morgan says, 

 are (1) that selection can not actually 

 produce anything new and (2) that 

 selection after a while loses its effect. 

 These objections open up contro- 

 N'crsies which cannot here be ]jursued. 

 Adopting for tlie moment Dr. Morgan's 

 views. "The cjuestion still remains: 



* Sec "Mendclism up-to-date," a Review of "The Meelianisin of Mendelian Heredity," liy Mor- 

 gan, Sturtcvanl, Mulk-rand Bridnes. [oi knal ok Hi- rkditv. \'o1. \'II. i))). 17-23, January. 1'>H). 



