Atavism is the recurrence, or tendency to recur, to an ancestral type, peculiarity, or disease after its disappearance for one or more generations. It is intermittent heredity; reversion; "dating back."

The offspring may be affected by physical conditions of parents at the time of copulation, and also by the condition of the parent during pregnancy. It is believed that the first fruitful connection may sometimes have a marked influence on subsequent offspring. It is contended by a few that, if a full-blooded animal has been fruitful by reason of connection with a mongrel, thereafter the blood of the dam is vitiated and she becomes incapable of producing full-blood offspring when united with a full-blood male of her own breed.

The relative age and vigor of parents may have a marked influence on the offspring. Animals which are overfat tend to lose somewhat the power of transmitting their highly specialized qualities and full vigor. In fact, animals overfat may be said to be on the verge of disease. Exercise, or the lack of it, in the parents may be a potent factor for good or evil, in reproduction.

In highly organized animals, mental impressions at time of copulation, and for some time afterwards, may have a marked influence on the offspring. In most cases, however, mental impressions play little or no part in the breeding of domestic animals.