Seoors fects of Rediation (fr. Belianr’) vulculetions show that mutstion rate within temperature ranges in which organisms aid Unrive, increases (in Drosophila) about two te five times for each 10 degree rise ge yo dive in tenperuture. One of the most puzzling proverties of the gene, at Least to the theoretical phesicist, is the stebility of the gene--a frope.t7 of great practical as well as theerelical importance. Since the diameter of the gene is almost certainly bebween tro and 10 millimi cron this means that a gene involves in the order of a million or co etoms. In teras ct statistical mechanics this smell number af atoms sheuld exhibit a ver ivceguier benevior., However, as such things co in biclezy, the gene represents one of the 2@ configuretions known. Tict even methemstiecal phrsicists mecard the gore es an interesting object and cereest Lllcevurctes tae fect that the Lounderice belvcen tne scversi selences cre beconin ST yt “a7, LadeCel ied unu @iifuss, The to At, sok ~ 2 wee tome om Tad Se} aa ne Thet Llane there should te reciproecsl couwributions betuesn medicine 20 gc. Tuscs is not curprising. Bot that geuctics se} suld srise to the point where it see fa te ternich a livtie the pright sanctity of en limertent concest in theoretisal physies i little startling. (Hamely, by wesenting the y.z2ie of expleining how so smell an amount of material eovid be so statle.} The secon part of this lecture will bo dseveved to natations, about which enous! information has been accumulated so thet we can classify veriaiions irto seversl cetegor: (1) Modifications—variations in the exgression of a character due to function: a yrocess in the organism or response tc the enhvironuent. Every character of the organism gene determined or not, responds developmentally to function and to environment. (2) Gene recombinations--which accourt for the mititade of relatively small ciraracteristics by means of which we distinguish nembers of the same species or of the « seme faally from each other.