code: def save_genome_fitness(self,
                              delimiter=' ',
                              filename='fitness_history.csv',
                              with_cross_validation=False):
          
          with open(filename, 'w') as f:
              w = csv.writer(f, delimiter=delimiter)
  
              best_fitness = [c.fitness for c in self.most_fit_genomes]
              avg_fitness = self.get_fitness_mean()
  
              if with_cross_validation: # pragma: no cover
                  cv_best_fitness = [c.cross_fitness for c in self.most_fit_genomes]
                  cv_avg_fitness = self.get_average_cross_validation_fitness()
                  for best, avg, cv_best, cv_avg in zip(best_fitness,
                                                        avg_fitness,
                                                        cv_best_fitness,
                                                        cv_avg_fitness):
                      w.writerow([best, avg, cv_best, cv_avg])
              else:
                  for best, avg in zip(best_fitness, avg_fitness):
                      w.writerow([best, avg])
docs: Saves the population's best and average fitness.