A process in nature in which organisms possessing certain genotypic characteristics that make them better adjusted to an environment tend to survive, reproduce, increase in number or frequency, and therefore, are able to transmit and perpetuate their essential genotypic qualities to succeeding generations.
A watchglass can be used as a beaker cover, but you can also place small amounts of chemicals on it and then heat it (for example, to evaporate some water from a wet chemical). From Wikipedia: A watch glass is a circular, slightly concave piece of glass used in chemistry as a surface to evaporate a liquid, or as a cover for a beaker. The latter use is generally applied to prevent dust or other particles entering the beaker; the watch glass does not completely seal the beaker, and so gas exchanges still occur.
It must also be able to withstand being dropped, or set down roughly. Thus, glass is not a good option for a watch cover. Though glass provides a transparent view and lovely finish, it is not very durable. Rolex has combated this issue, using two varieties of crystals over the years in the production of their famous, luxury watches. Up until the late 20th Century, most watch crystals were actually acrylic like the one in the image above.