630 



NATURE 



[January 13, 192 1 



attain this end the relation between the water levels 

 outside and inside the dam must be that shown in 

 Fig. I by the full and dotted lines respectively. The 

 motors or turbines must be designed to work efficiently 

 with any head from o to H, and able to do 

 all the work in such a short time that the high and 

 low water levels remain practically constant during 

 that interval, say 20 minutes. 



If the interval between high and low water is six 

 hours, the motors must be capable of delivering 

 eighteen times the average power, and of this power 

 17/18 must be stored. It is, of course, impracticable 

 to fulfil these conditions. The turbines have to work 

 with a nearly constant head, and it would be im- 

 possible to arrange for the whole work to be done in 

 the short time available at high and low water. 



The most practicable plan is indicated in Fig. 2. 

 The flow through the turbines is adjusted to reduce 

 the rise and fall inside the dam to half that of the 



Fig. I. Fig. 2. 



tide and the time of working increased to about three 

 hours. The effective head is H/4, and the distance 

 through which it acts (i.e. the stroke) is H/2. In 



this way about one-fourth of the tidal energy I iv.K — 1 



might be utilised, half of which would have to be 

 stored if the power supply is to be constant. Taking 

 into account the various losses due to turbine, elec- 

 trical, and storage efficiency, it is improbable that 

 more than one-fifth of the whole tidal power could be 

 delivered as electric current. 



If A is 1 square mile and H 30 ft., it will be 

 foimd that for each square mile of reservoir surface 

 something more than 10,000 h.p. might be expected. 



I have no knowledge of the details of the Severn 

 scheme, but if it were possible — which I doubt — to 

 enclose 20 square miles of estuary where the average 

 difference of tidal level was 30 ft., the power avail- 

 able for distribution would be under 250,000 h.p. 



It may be remarked that the same power could be 

 obtained from a river having a current of little more 

 than 2 miles per hour and a cross-sectional area of 

 2000 ft., if in its course there was a fall of 30 ft. 



A. Mai.i.ock. 



Heredity and Acquired Characters. 



In Nature of January 6 there appears a long com- 

 munication on heredity by Sir .\rchdall Reid which 

 he conceives to be. a reply to criticisms made on a 

 former letter by him on the same subject by .Sir Rav 

 Lankester, Prof. Poulton, Dr. Gate.s, and mvself. 

 Leaving these eminent biologists to look after them- 

 selves, which they are quite capable of doing, perhaps 

 vou will allow me to say a word or two on some 

 points raised in the letter in_ the current issue. 



Sir Archdall Reid accuses me of torturing "a word 

 which has now an established and perfectly clear 

 meaning." The word is "variation." I wished to 

 contribute to clearness by defining it, for, so far from 

 its having a clear meaning, there are at least three 

 senses in which it can be used. Further, let me sav 

 that if a five-fingered child were born of a six-fingered 

 parent, I should not describe it as a "variation," but 

 as a "reversion." 



Next, Sir .\rchdall Reid challenges me to define the 

 NO. 2672, VOL. 106] 



"quibble" about "acquired character." "Acquired 

 character " is a technical term ; by it is meant a 

 quality, i.e. the degree of development of an organ, 

 which is produced as a response to function, altered 

 from the normal in response to an alteration of the 

 environment from the normal ; but Sir .Archdall Reid 

 interprets it as any adult character whatever. 



Sir .Archdall Reid has, howiever, understood the 

 point, because he says that " the supposition that 

 'acquirements ' tend to become ' innate ' is . . . 

 ridiculous." Such an ex cathedra statement contributes 

 nothing useful to the discussion. There are definite 

 experiments on record which, if true, prove this very 

 point, but Sir .\rchdall Reid apparently knows nothing 

 about them. 



Then we are told that "low in the animal scale we 

 find little or no evidence of development in response to 

 functional activity." On reading this the question 

 that instantly occurs to one's mind is: "Where did 

 Sir Archdall Reid learn his zoology?" I have been 

 working with Echinoderm larvse for many years, and 

 in no animals known to me is structure more sensi- 

 tive to changes in the environment (Proc. Rov. Soc, 

 ^, vol. xc, 1918). 



Lastly, Sir Archdall Reid says : " We are now in 

 the morass in which Lamarck and Weismann floun- 

 dered." I have attended many congresses of bio- 

 logists, and I have never found evidence of confusion 

 in their minds as to what was meant by an " acquired 

 character." They differed, and continue to differ, as 

 to whether there is evidence that an "acquired 

 character " can be inherited, or, to use Sir Archdall 

 Reid's paraphrase, that "acquirements" can become 

 "innate," and this difference can be settled only by the 

 outcome of experiments which are now in progress, 

 but I have nowhere detected evidence of a condition 

 of thought that could be described as a "morass." 

 I conclude, therefore, that it exists alone in Sir 

 Archdall Reid's mind. E. W^ MacBridf.. 



Imp)erial College of Science and Technology, 

 South Kensington, S.W.7, January 7. 



Provided that biologists understand one another, 

 it is, perhaps, not an insuperable barrier to the pro- 

 gress of biology that .Sir .Archdall Reid is unable to 

 understand their terminologx . I write merely to point 

 out that though he seeks to teach biologists the 

 proper use of terms, Sir .Archdall Reid, in his letter 

 in Nature of January 6, contradicts himself in 

 his own terminology. He states that even in 

 human beings many characters do not develop in 

 the least in response to functional activity, e.g. hair 

 and external generative organs. On the other hand, 

 in man most characters develop wholly, or almost 

 wholly, in response to that stimulus. Yet in another 

 paragraph he asserts that all characters are neces- 

 sarilv innate, acquired, germinal, somatic, and inherit- 

 able in cxacWy the same sense and degree. If bio- 

 logists recognise, as Sir .Archdall Reid does, a differ- 

 ence between characters that develop in response to 

 functional activitv and those which do not, what need 

 is there for him to ask biologists why they describe 

 some characters as "innate," "germinal," and "in- 

 heritable," and others as "acquired," "somatic," and 

 "non-inheritable"? J. T. Cunnincii.«i. 



L'niversity of London Club, 21 Gower 

 Street, W.C.i, January 7. 



Solar Radiation in Relation to the Position of Spots 

 and Faculse. 



.About September i last an arrangement was made 

 between the Director of the .Argentine Meteorological 

 Service and the Director of the .Astronomical Observa- 

 tory of the University of La Plata for observations of 



