Received for publication 13 Oct. 1988. This work was supported in part by the Fulbright Commission of the United States and CAPES (the Brazilian Fulbright agent) for a sabbatical grant to A.G. and by the continuous A.R.T. funds from Wm. Paterson College. We are grateful to John Hewitt for statistical analysis and to Robert F. Callahan for the critical review of this manuscript. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact.