Batavin Lactuca sativa L. var. capitata

Unlike the chicory salads, the various types of lettuce belong to the lettuce (Lactuca) Genus. The lettuce family includes heading lettuce (both butterhead and crisphead varieties), Batavian lettuce (an intermediary form between the two), Romaine lettuce and loose leaf lettuces with various shapes and colours (Oak leaf, Lollo Rossa etc.). Romaine lettuce was probably cultivated in a similar form by the Ancient Romans making it the original cultivated type.
Baby leaf salad is not a special botanical variety. The leaves of various lettuce varieties but also endives, beetroot, spinach and many cabbage varieties are cut and harvested young.

Cultural tip:
Lettuce plants are not able to heal themselves. Soil borne infections such as bottom rot gain access via damaged leaves and older leaves that lie on the ground. Young plants should therefore be small enough when being planted that they are not damaged. The root ball should only be planted to half its depth so as to minimise leaf contact with the soil.
Optimum seed germination temperature is between 15 - 18 °C. Depending on variety anything above 20° C can inhibit germination. When covering the lettuce bed it should be remembered that lettuces won't produce heads in temperatures above 30 °C.

Sources:
VOGEL, G., 1996: Handbuch des speziellen Gemüsebaues. Verlag Eugen Ulmer.
WEBERLING, F. & H. O. SCHWANTES, 1987: Pflanzensystematik. Verlag Eugen Ulmer.
GEISLER, G., 1991: Farbatlas Landwirtschaftliche Kulturpflanzen. Verlag Eugen Ulmer.