Spines and prickles may often disappear and give rise to unarmed and defenceless types. Of the thorn-apples both species, the whiteflowered _Datura Stramonium_ and the purple _D. Tatula_ have such varieties. Spinach has a variety called the "Dutch," which lacks the prickles of the fruit; it is a very old form and absolutely constant, as are also the thornless thorn-apples. Last year a very curious instance of a partial loss of prickles was discovered by Mr. Cockerell of East Las Vegas in New Mexico. It is a variety of the American cocklebur, often called sea-burdock, or the [140] hedgehog-burweed, a stout and common weed of the western states. Its Latin name is _Xanthium canadense_ or _X. commune_ and the form referred to is named by Mr. Cockerell, _X. Wootoni_, in honor of Professor E.o. Wooton who described the first collected specimens. The burs of the common species are densely covered with long prickles, which are slightly hooked at the apex. In the new form, which is similar in all other respects to the common cocklebur, the burs are more slender and the prickles much less numerous, about 25 to the bur and mostly stouter at the base. It occurs abundantly in New Mexico, always growing with the common species, and seems to be quite constant from seed. Mr. Cockerell kindly sent me some burs of both forms, and from these I raised in my garden last year a nice lot of the common, as well as of the _Wootoni_ plants. Spineless varieties are recorded for the bastard-acacia, the holly and the garden gooseberry (_Ribes Grossularia_, or _R. Uva-crispa_). A spineless sport of the prickly Broom (_Ulex europaeus_) has been seen from time to time, but it has not been propagated. Summarizing the foregoing facts, we have excellent evidence of varieties being produced either by the loss of some marked peculiarity or by the acquisition of others that are already [141] present in allied species. There are a great many cases however, in which the morphologic cause of the dissimilarity is not so easily discerned. But there is no reason to doubt that most of them will be found to conform to the rule on closer investigation. Therefore we can consider the following as the principal difference between elementary species and varieties; that the first arise by the acquisition of entirely new characters, and the latter by the loss of existing qualities or by the gain of such peculiarities as may already be seen in other allied species.