This court has also generally held that children of tender years, particularly girls, should be awarded to the custody of the mother unless she is morally unfit. Fisher v. Fisher, 133 Or. 318, 320, 289 P.2d 1062; Baier v. Baier, 172 Or. 83, 87, 139 P.2d 562; Leverich v. Leverich, 175 Or. 174, 177, 152 P.2d 303; Richardson v. Richardson, 182 Or. 141, 143, 186 P.2d 398; Ruch v. Ruch, 183 Or. 240, 244, 192 P.2d 272; Cripe v. Cripe, supra; Goldson v. Goldson, 192 Or. 611, 621, 236 P.2d 314; and Wilson v. Wilson, 199 Or. 263, 260 P.2d 952. This salutary rule should be followed whenever possible. However, it is not controlling and should be applied only when its application will be in the best interests of the children.

The necessity for such change in this case, if any, must have become evident between February 13, 1953, the date of the decree, and October 2, 1953, the date when the motion for modification was filed. Wilson v. Wilson, 199 Or. 263, 266, 260 P.2d 952; Sakraida v. Sakraida, 192 Or. 217, 226, 217 P.2d 242, 233 P.2d 762; Leverich v. Leverich, 175 Or. 174, 179, 152 P.2d 303. The burden of showing a change of conditions warranting a change of custody rests upon the party seeking the modification, in this case upon the mother of the child. Gibson v. Gibson, 196 Or. 198, 213, 247 P.2d 757; Leverich v. Leverich, supra, at page 180; Kellogg v. Kellogg, 187 Or. 617, 621, 213 P.2d 172.