HARDWICKE'S SCIENCE-GOSSIP. 



24s 



own box, 3 in. deep, holds 24 of these trays, capable of 

 accommodating 288 slides, and the cost of the whole 

 is between 4J. and ^s., or less than one-fifth for the 

 same amount of accommodation in boxes sold for the 

 purpose. 



The cardboard and millboard must be cut with a 

 knife, not with scissors ; if gum is used it must be 

 well dissolved, strained and very thick, and each tray 

 as it is made must be placed under a weight, one on 

 the top of another. If the thickness of the millboard 

 is not enough to furnish depth for slides containing 

 objects mounted in raised cells, it can be increased 

 by adding a thickness of cardboard, or even another 

 of millboard. 



W. P. Hamilton. 



Shrrdusbiiry.. 



VARIATION IN THE MOLLUSCA, AND ITS 

 PROBABLE CAUSE. 



By Joseph W. Williams. 



Part IV. — The Probable Cause of Variation. 



" O mother and queen, beneath the olden spell 

 Of silence, gazing from thy hills and skies ! 

 Dumb mother, struggling with the years to tell 

 The secret at thy heart through helpless eyes ! " 



Theodore Watts in " Natura Benigna." 



I WISH the reader to take especial notice that I 

 undogmatically and advisably head this chapter 

 as dealing with the probable, not the, cause of varia- 

 tion. Had I done otherwise, I, indeed, had taken a 

 false step, since it is certainly not without the bounds 

 of probability that the whole subject of heredity may 

 take, in the course of further work, a different aspect 

 and a different interpretation. I say it may take that 

 course, but I will not say that it will ; at present it 

 is but a theory which, although presenting a large 

 amount of plausibility, yet, as a theory, dogmatism 

 in connection with it is out of place. The reader, 

 therefore, must not consider me as dogmatic on any 

 one point I introduce. 



I divide variation up into two distinct groups, and 

 I think rightly. On one of these my attention will be 

 mostly occupied, which I call congenital variation, 

 that is to say, variation present in the animal pre- 

 natally or natally, either actually or latently ; as 

 opposed to the other, acquired variation, due to varia- 

 tion acquired after birth, as by force of circumstances, 

 which I shall touch but lightly upon. That an albino 

 parent shell can have an albino offspring, as has been 

 recorded by Colbeau* (though this does not always 

 obtain) must be rendered under my division, con- 

 genital variation ; that in a normally unicolorous shell 

 a pale-coloured band is sometimes present which has 

 been stated by Hazayf to be due to a wound of the 

 mantle-edge destroying the unicellular glands in that 



* "Bull. Soc. Mai. Belg.," vii. p. Ixxxix. 

 t "Mai. Blatt." (2), iv. pp. 103-105. 



region, must be regarded as a case of acquired c/r. 

 accidental variation, and so on ; but these two cases - 

 are enough to render my divisions clear to the mind 

 of the reader. 



I consider heredity as the principal and mastei^ 

 factor in producing cases of variation which may be 

 legitimately considered as of congenital origin. In 

 them the principle of adaptation has had an opposing 

 force, I must admit ; but its opposing force has been 

 of a minor character. Gasquet has compared the law 

 of heredity to Newton's first law of motion, which may 

 be tersely expressed that "matter has inertia," or, in 

 Newton's own words, *' Every body tends to persevere 

 in its state of rest or of uniform motion in a straight line, 

 unless in so far as it is acted on by impressed force." 

 Wiedersheim, in his "Grundriss der Vergleichenden 

 Anatomic der Wirbelthiere," states that " the capacity 

 of adaptation varies inversely with the persistence of 

 inherited qualities. These two opposing factors, 

 adaptation and heredity, constitute the formative 

 principle of the animal body." 



What, then, is the state of our knowledge with 

 regard to the principle or law of heredity in all its 

 intricacies and complications? Can we bring all 

 that has been written upon it down to a substantial 

 and tentative whole? That the whole subject can 

 be reduced to one thing — -the plastic nature of proto- 

 plasm — does not admit of a doubt ; but such a simple 

 statement does not satisfy our inquisitive longing for 

 a thorough knowledge of what, a few years back, was 

 one of Nature's greatest arcana, and we must there- 

 fore go deeper into the subject. I have already, in 

 part III., sketched, as fully as my space would admits 

 the various phases of the cell theory, and in this 

 paper I shall use the terms there explained withouir 

 stint. It is hoped that the reader interested in this, 

 question has already made himself familiar with them 

 in their entirety. 



It is not a question of psychology, and I shall there- 

 fore pass by the theories of Stahl and others, whicii^ • 

 are " explanations without an explanation." It is a 

 question of hard fact upon which to build our theories, 

 and hard fact we want. Charles Darmn * tried to 

 give an explanation by his theory of pangenesis, and 

 Ernst Hack elf has tried with his theory of perigenesis y 

 but these theories are rather too conjectural for our 

 practical needs. The latter theory fails because it 

 brings in to our acceptance an assumption of which we 

 know as yet practically nothing. We know practic- 

 ally nothing of molecular movements. In his thesis- 

 on this subject, Hackel argues that some kind of mole- 

 cular movement is transmitted which the tissues have 

 acquired by constant repetition ; that heredity is 

 dependent upon a true and accurate transmission of 

 distinct species of movements of molecules, and that 



* "Variation of Plants and Animals under Domestication," 

 1867, Chaps. 37 and 38; also Herbert Spencer's "Principles of 

 Biology," Vol. I, Chaps. 4 and 8. 



t " Die Perigenesis der Plastidule, oder die Welienzeugung , 

 der Lebenstheilchen," 1876. 



