Page:Popular Science Monthly Volume 30.djvu/655

Rh and its relative importance determined, before conclusions can be drawn, and even then it is frequently necessary to estimate the relative probability of two or more alternatives, thus giving an exercise to the powers of ratiocination which is utterly lacking in the remorseless logic of the mathematical theorem.

When this general foundation of facts and methods is obtained, more special studies are taken up; and since embryological research includes most of the processes involved, we will suppose that the student is next introduced to this fascinating field which is now so assiduously cultivated by scholars all over the world. As a rule, it may be stated that animals living on the land or in fresh water differ considerably in their mode of development from their near relatives in the sea. Many ancestral features which are retained in marine forms have become eliminated in the others, and hence the study of the growth of salt-water forms from the egg to the adult throws far more light on the relationships and ancestry of the different groups than does that of the terrestrial and fluviatile species. Again, the sea affords a wealth of life far beyond that of the land and fresh water, a wealth to be estimated not only in number of individuals but of species as well. Whole groups of animals are solely marine, while others are represented on the earth or in rivers and ponds by a few small and insignificant forms.

For this reason the student of embryology betakes himself every year to the shore, so that, while being recuperated by the sea-breeze, he may continue his studies and add to the total of human knowledge. Marine laboratories for this purpose are scattered the whole length of our coast, from North Carolina to Eastport, some being mere temporary affairs, others permanent stations. In 1885 public or private laboratories existed at Beaufort, Newport, Nantucket, "Wood's Holl, Salem, Annisquam, Mount Desert, and Eastport. The absolutely necessary furnishings of such a laboratory are extremely few, but to them one may add as far as purse and inclination admit. There must be tables, chairs, and glass dishes, while each student must have a microscope and accessoires. Then come a boat (a row-boat is sufficient for all ordinary work) and apparatus for collecting. Finally, a small stock of chemicals and apparatus for microscopical work complete the list of necessities.

Possibly the most common way for obtaining material for embryological study is by use of the skimming-net. This consists of a brass ring about a foot in diameter, to which is attached a net of fine gauze and cords for dragging it behind a boat. The whole operation of skimming is very simple. Two persons are required, one to row the boat, the other to attend the net. The latter allows the net to trail behind, keeping the cords so that part of the mouth is above and part below the surface of the water, so that as much as possible of the surface-water will be strained through the gauze. At intervals the net is