of _Panmixia_,--_all_ individuals being equally free to survive
and commingle their variations, and not merely selected or favoured
individuals. See his _Essays on Heredity_, &c., p. 90 (Clarendon Press).

[4] Inclusive in each case of fixed strengthening wire weighing about a
sixteenth of an ounce or less.

[5] References of course are to _Factors of Organic Evolution_.

[6] P. 13; and _Nineteenth Century_, February, 1888, p. 211.

[7] Tomes's _Dental Surgery_, pp. 273-275. Tomes observes that it is as
yet uncertain in what way civilization predisposes to caries. But he
shows that caries is caused by the lime salts in the teeth being
attacked by _acids_ from decomposing food in crevices, from artificial
drink such as cyder, from sugar, from medicine, and from vitiated
secretions of the mouth. It is evident that in civilized races natural
selection cannot so rigorously insist on sound teeth, sound
constitutions, and _protective alkaline_ saliva. The reaction of the
civilized mouth is often acid, especially when the system is disordered
by dyspepsia or other diseases or forms of ill-health common under
civilization. The main supply of saliva, which is poured from the cheeks
opposite the upper molars, is often acid when in small quantities. But
the submaxillary and sub-lingual saliva poured out at the foot of the
lower incisors and held in the front part of the jaw as in a spoon,
"differs from parotid saliva in being more alkaline" (Foster's _Text
Book of Physiology_, p. 238; Tomes, pp. 284, 685). One observer says
that the reaction near the lower incisors is "never acid." Hence (I
conclude) the remarkable immunity of the lower incisors and canines from
decay, an immunity which extends backwards in a lessening degree to the
first and second bicuspids. The close packing of the lower incisors may
assist by preventing the retention of decaying fragments of food. Sexual
selection may promote caries by favouring white teeth, which are more
prone to decay than yellow ones. Acid vitiation of the mucus might
account both for caries and (possibly) for the strange infertility of
some inferior races under civilization.

[8] _Origin of Species_, pp. 198-9; _Variation of Animals and Plants
under Domestication_, vol. ii. p. 328 footnote, also p. 206.

[9] Mr. Spencer weakly argues that an advantageous attribute (such as
swiftness, keen sight, courage, sagacity, strength, &c.) cannot be
increased by natural selection unless it is