HA RD WICKE'S SCIENCE- G OS SIP. 



179 



those few who have published their thoughts on this 

 matter. Those few, however, have published an 

 explanation that the evolution of the shell-colour has 

 proceeded along a line leading from the complex to 

 the simple, which seems to me as somewhat contrary 

 to the law of nature, while what I believe nature has 

 been telling us all along is that evolution of all kinds 

 — the shell-colour as well as the mammal from the 

 simple protoplast — has differentiated along a line 

 exactly the polar opposite of that — from the simple 

 to the complex. And I believe it to be as true in the 

 organic world that " ontogeny is a short repetition of 

 phylogeny" as Kepler's law in relation to the 

 motions of the planetary orbs, that "the radius vector 

 of each planet describes equal areas in equal times,"- 

 or Newton's law of gravitation in the physical world. 

 Upon these two facts of nature, then — that evolution 

 has proceeded along a line leading from the simple to 

 the complex, and that " ontogeny is a short repetition 

 of phylogeny " — I shall build the theory that the 

 primitive colour of our land and freshwater shells 

 was horn-coloured, and that the bands first arose as 

 points which afterwards coalesced ; the second of 

 these facts, however, involves the first, and may be 

 stated in other words, that the development of the 

 shell is an epitome of its ancestry. I am also 

 inclined to the belief that the colour to be produced 

 after the horn was white or whitish, and this for 

 reasons which will be presently given ; the other 

 colours may, perhaps, have been produced by 

 combinations ; but this does not concern my present 

 theory. 



The facts which seem to me to support such a 

 theory as the one advanced are as follows : — 



1. In the development of the snail, during what is 

 known as the trochosphere-stage, a thickening of the 

 epiblast on the posterior and dorsal side of the larva 

 obtains, termed the shell-gland. This soon in- 

 vaginates, and a chitinous horn-coloured plug may 

 become developed in it, and especially so, according 

 to Balfour, in abnormal larvae. This plug, or 

 primary shell, becomes absorbed except in clausilia, 

 arion, amalia, and limax, persisting in the first as the 

 nucleus, and in the last three as the internal shell. 



2. During the next stage of development, called 

 by Lankester the veliger-stage, and characterised by 

 the formation of the foot and velum, the shell-gland 

 becomes everted to form a disc-like area which 

 becomes the mantle, and on which the secondary or 

 permanent shell is secreted. This secondary shell is 

 always at first horn-coloured or whitish (generally 

 the first), and always bandless. 



3. The majority of freshwater shells are horn- 

 coloured and bandless, and the non-development of 

 colour may be explained on the ground, and 

 legitimately, that environmental conditions are not so 

 great as on land. 



4. In these horn-coloured freshwater shells an 

 advance in colour is often seen by the development 



of white flammules. This I have seen in L. stagnalis, 

 L. percgra, and L. auricularia among our species, 

 and an inspection of the collections in the National 

 Museum will reveal many among foreign specimens. 

 Apart, entirely, from the development of these white 

 flammules, completely albino specimens occur, as 

 Physa fontinalis, var. albino, (Jeff.), Limncca peregra, 

 var. Candida (Porro), Limncca auricularia, var. 

 albida (Jeff.), and Limniza palustris, var. albida 

 (Nelson). 



5. The nucleus of the secondary shell is always 

 white, whitish, or horn-coloured, and scarcely ever 

 banded. 



6. Considering the large degree of environmental 

 conditions on land, some varieties of land shells, 

 backed by their developmental history as concerns 

 colour, may be legitimately regarded as reversions to 

 ancestral conditions. Such would I regard H. aspersa, 

 var. exalbida (Menke), H. nemoralis, var. albescens 

 (Moq.), H. horlensis, var. albina (Moq.), H. hortensis, 

 var. arenicola (Macgill), H. hortensis, var. pallida 

 (Ckl.), H. arbustorum, var. albinos (Moq.), H. 

 cantiana, var. albida (Taylor), H. rufescens, var. alba 

 (Moq.), H. concinna, var. albida (Jeff.), II. granulata, 

 vars. cornea and albida, H. pisana, var. alba (Shuttl.), 

 H. virgata, var. pelluscens (Shuttl.), var. albicans 

 (Gratel), H. caperata, var. alba (Pic), H. ericetoriim, 

 var. alba (Charp.), var. cornea (Loc), var. vitrea 

 (D. & M.), H. rotundata, var. alba (Moq.), H. 

 lapicida, var. albina (Menke), H. acuta, var. alba 

 (Req.), B. montanus, var. albina (Moq.), B. obscurus, 

 var. albinos (Moq.), P. umbilicata, var. albina (Moq.), 

 and so on. 



7. That Cyclosfomaelegans, being a pneumochlamyd, 

 is an example of a freshwater form which has 

 adapted itself to live on land, and possesses the 

 following suggestive varieties, viz., var. albescens 

 (des Mouls), var. ochroleuca (des Mouls), and var. 

 pallida (Moq.). 



8. That even noting the large degree of environ- 

 mental conditions on land, yet many are horn- 

 coloured in their typical condition, as instance the 

 hyalinre, and the amphibious succinece. That the 

 commonest colour-variation in these is white, as 

 instance S. elegans, var. albida (Taylor), H. cellaria, 

 var. albinos (Moq.), H. nitidula, var. helmii (Alder), 

 H. pur a, var. margaritacea (Jeff.), H. nitida, var. 

 allmios (Moq.), etc. 



9. That considering progression has been from the 

 simple to the complex, is prima facie evidence that 

 bands first were points which afterwards coalesced 

 even if there were no other evidence forthcoming. 

 But what may be legitimately considered as 

 " reversions " occur, as H. aspersa, var. flammea. 

 (Pic), H. nemoralis, var. interrupta (Moq.), H. 

 hortensis, vars. intemipta and picnctclla, H. virgata, 

 var. tessellata (Bouch.), and so on. 



10. That even considering the fact that horny and 

 white coloured varieties of our land specimens occur, 



