Type parameter question

question.scala

trait Indiv[V <: Indiv[V]] {
  val fitness: Double
  def reproduce(other: V): V
}

class RealIndiv[V](override val fitness: Double) extends Indiv[V] {
  override def reproduce(other: V) = new RealIndiv(fitness + other.fitness)
                                                                  // ^ Cannot resolve symbol fitness (on `other.fitness`)
}