Selected Reform Responsa 139
We should also note that the castration of animals was prohibited, and this has always been considered as a form of maiming, which was forbidden(Shelat Yaabetz 1.11). 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 a great interest in maintaining species of both plants and animals separately, 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 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 with the cross-breeding of certain species of animals or plants. The Mishnah and Tosefia 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.