Kindness to animals included the lightening of the load from an overburdened animal(Ex. 13:5). Domestic animals were required to rest on Shabbat as human beings were(Ex. 20:10; 23.12; Deut. 5:14). Provisions were made for animal care on Shabbat , an animal that was normally milked by a non-Jew . If an animal needed to be rescued, it was to be done even on Shabbat (Shab. 128a; Yad, Hil. Shabbat 25:26; Shulhan Arukh, Orah Hayyim 305:19.
We should also note that the castration of animals was always prohibited and considered as a form of maiming, which was forbidden(Shelat Yaabetz 1.:1). We may summarize this by relating that our tradition demands kind treatment of animals. They may be used by human beings but not treated cruelly. We should note that the medieval discussion by some Jewish philosophers about the soul of animals was left as a speculative issue.
Now let us deal with genetically induced changes in mice that are to be used as experimental animals. Systemic genetic changes are a recent scientific achievement. The only area that approached this field in the past was controlled breeding. Our tradition had very little to say about breeding animals, as long as no attempt was made to do so with unlike species. There was great interest in maintaining separate species of both plants and animals, based in part on biblical verses(Lev. 19:19; Deut. 22:10). An entire section of the Mishnah (Kilaim) dealt with the problem of sowing various kinds of seeds together, grafting one plant onto another, and the interbreeding of animals. This segment of the Mishnah contains eight chapters that dealt with various kinds of mixtures such as the prohibition against interweaving wool and linen and the crossbreeding of certain species of animals or plants. The Mishnah and Tosefta Kilaim indicated a fascination with mixtures and sought to explain the natural world from this perspective. The Mishnah Kilaim presented two points of view, according to a recent scholarly volume by Avery-Peck. The circle of Yavneh argued that species were to be kept separate, as God created order in the universe and it was Israel 's duty to maintain this separation.
Those of Usha argued that Israel imposed order onto the natural world and Israel now had to maintain it. Neither group ultimately included non-edible plants in their scheme.(A.]. Avery-Peck, The Mishnah’s Division of Agriculture[Chico , CA: Scholars Press, 1985])