The N. Y. Evening Post tlius speaks of Gen. Cass'-., recent .-.letter agaii.st the Wilmot Proviso. ' jjjs" We are sorry to pee ttiat Genera! Cnss, in this question of the prohibition of slavery, hns been slipping from stnge lo stage in ;i very odd mnnner. lic was firsÃ a friend of ihe Wilmot Proviso; then he thoÃ¼glÃt the Wilmoi Proviso.thnt is lo say, tlie excliisiofl of slavery from the new len-itory, perfeotly right Ãn principie, bilt on] y premature intime; now he holds ihat Congress has no right to prohibid slavery in ihe territories. There is a story of a Vermorter who, being sued at law fur damagfs In brcaking an iron kettle whicli he liad borro wed. pleaded in court, fir.-t, that he never liad the kettle; secondly, ihat it was broken wrren he borrowed it ; nnd Ihirdly, that it was wh -Ie when he returned Hi Gen. Cass was in 1846 for thr WÃ¼mot Proviso - he never hnd the kellle - in 1847 the Wilmot Proviso was right in principie, but wrong in ihe time of is application - the kettle was broke when he borrowod it - and now, in 1848, the Wilmot Proviso is unconstitutional and unnecesssary - ihe kettle was whole wheu he returned it. So it is with all the prematuri.sts - warm friends n yeasinceofthe principie of excluding .lavefy-they have come al last to the convenient conclusiÃ³n, when the prelext of prematurity will serve them no longer, that the Constitution gives Congress no aulhority td pfovide for its prohibition.